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<div id="triangle-up"></div>

@mskfisher The size of the bottom border in your equilateral triangle shouldn't be ceil(sqrt(3) * 60) ? A few pixels more then! – Niloct Aug 17, 2011 at 12:22 @Niloct: I eyeballed it quickly at round numbers. You're right, the more accurate equilateral measurements would be (side:58,bottom: 100) or (side:60,bottom:104). – mskfisher Aug 17, 2011 at 12:41 Here's a great CSS triangle generator for every directions and sizes: apps.eky.hk/css-triangle-generator – Allan Stepps Mar 7, 2014 at 23:40 @Jauzsika, you can add these triangles into a page without adding additional elements simply by using :before or :after pseudo classes. – zzzzBov Aug 16, 2011 at 13:24 Act with animations: jsfiddle.net/pimvdb/mA4Cu/104 . Just for those of us who need even more visual proof like me... – pimvdb Aug 18, 2011 at 20:26 With different left-border and right-border , non-isosceles triangle can be made. And when many triangles are combined... jsfiddle.net/zRNgz – JiminP Aug 26, 2011 at 2:47

Start with a basic square and borders. Each border will be given a different color so we can tell them apart:

.triangle {
    border-color: yellow blue red green;
    border-style: solid;
    border-width: 200px 200px 200px 200px;
    ;
    width: 0px;
<div class="triangle"></div>
which gives you this:
But there's no need for the top border, so set its width to 0px. Now our border-bottom of 200px will make our triangle 200px tall.
.triangle {
    border-color: yellow blue red green;
    border-style: solid;
    border-width: 0px 200px 200px 200px;
    ;
    width: 0px;
<div class="triangle"></div>
and we will get this:
Then to hide the two side triangles, set the border-color to transparent. Since the top-border has been effectively deleted, we can set the border-top-color to transparent as well.
.triangle {
    border-color: transparent transparent red transparent;
    border-style: solid;
    border-width: 0px 200px 200px 200px;
    ;
    width: 0px;
<div class="triangle"></div>
finally we get this:
                There's another way to draw .., which turns out to be the same way :) Nice explanation though
– TheTechGuy
                Aug 17, 2011 at 19:21
                -1 for using JPEGs with massive artifacts. But +1 for creating a great example of when not to use JPEGs that I can link to in the future for deterrance. ;)
– Henrik Heimbuerger
                Aug 18, 2011 at 8:24
                Sorry @hheimbuerger, I messed up your example by fixing the images. You’ll have to link to revision 2 of this answer in the future.
– Rory O'Kane
                Aug 15, 2012 at 20:05
Different approach:
CSS3 triangles with transform rotate
Triangular shape is pretty easy to make using this technique. For people who prefer to see an animation explaining how this technique works here it is :
Link to the ANIMATION : How to make a CSS3 triangle.
And DEMO : CSS3 triangles made with transform rotate.
Otherwise, here is detailed explanation in 4 acts (this is not a tragedy) of how to make an isosceles right-angled triangle with one element.
Note 1 : for non isosceles triangles and fancy stuff, you can see step 4.
Note 2 : in the following snippets, the vendor prefixes aren't included. they are included in the codepen demos.
Note 3 : the HTML for the following explanation is always : <div class="tr"></div>
STEP 1 : Make a div
Easy, just make sure that width = 1.41 x height. You may use any techinque (see here) including the use of percentages and padding-bottom to maintain the aspect ratio and make a responsive triangle. In the following image, the div has a golden yellow border.
In that div, insert a pseudo element and give it 100% width and height of parent. The pseudo element has a blue background in the following image.
At this point, we have this CSS :
.tr {
    width: 30%;
    padding-bottom: 21.27%; /* = width / 1.41 */
    position: relative;
.tr: before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: #0079C6;
STEP 2 : Let's rotate
First, most important : define a transform origin. The default origin is in the center of the pseudo element and we need it at the bottom left. By adding this CSS to the pseudo element :
transform-origin:0 100%; or transform-origin: left bottom;
Now we can rotate the pseudo element 45 degrees clockwise with transform : rotate(45deg);
At this point, we have this CSS :
.tr {
    width: 30%;
    padding-bottom: 21.27%; /* = width / 1.41 */
    position: relative;
.tr:before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: #0079C6;
    transform-origin: 0 100%;        
    transform: rotate(45deg);
STEP 3 : hide it
To hide the unwanted parts of the pseudo element (everything that overflows the div with the yellow border) you just need to set overflow:hidden; on the container. after removing the yellow border, you get... a TRIANGLE! :
CSS :
.tr {
    width: 30%;
    padding-bottom: 21.27%; /* = width / 1.41 */
    position: relative;
    overflow: hidden;
.tr:before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background-color: #0079C6;
    transform-origin: 0 100%;
    transform: rotate(45deg);
STEP 4 : go further...
As shown in the demo, you can customize the triangles :
Make them thinner or flatter by playing with skewX().
Make them point left, right or any other direction by playing with the transform origin and rotation direction.
Make some reflexion with 3D transform property.
Give the triangle borders
Put an image inside the triangle
Much more... Unleash the powers of CSS3!
Why use this technique?
Triangle can easily be responsive.
You can make a triangle with border.
You can maintain the boundaries of the triangle. This means that you can trigger the hover state or click event only when the cursor is inside the triangle. This can become very handy in some situations like this one where each triangle can't overlay its neighbours so each triangle has its own hover state.
You can make some fancy effects like reflections.
It will help you understand 2d and 3d transform properties.
Why not use this technique?
The main drawback is the browser compatibility, the 2d transform properties are supported by IE9+ and therefore you can't use this technique if you plan on supporting IE8. See CanIuse for more info. For some fancy effects using 3d transforms like the reflection browser support is IE10+ (see canIuse for more info).
You don't need anything responsive and a plain triangle is fine for you then you should go for the border technique explained here : better browser compatibility and easier to understand thanks to the amazing posts here.
                It's probably worth mentioning that the 1.41 is an approximation of √2 and the length of the hypotenuse of the triangle that you create, which is why you need (at least) that width.
– KRyan
                Feb 5, 2015 at 16:24
                Actually, in trying to use this, a mention of how the different widths for use with skewX were derived would be useful.
– KRyan
                Feb 5, 2015 at 17:06
                This is for the case you want to create borders for triangle, I was stuck with the borders way when I want to create this annotation box AnnotationBox
– vanduc1102
                Apr 4, 2017 at 15:56
transforms = [
         {'border-left-width'   :'30', 'margin-left': '70'},
         {'border-bottom-width' :'80'},
         {'border-right-width'  :'30'},
         {'border-top-width'    :'0', 'margin-top': '70'},
         {'width'               :'0'},
         {'height'              :'0', 'margin-top': '120'},
         {'borderLeftColor'     :'transparent'},
         {'borderRightColor'    :'transparent'}
$('#a').click(function() {$('.border').trigger("click");});
(function($) {
    var duration = 1000
    $('.border').click(function() {
		  for ( var i=0; i < transforms.length; i++ ) {
        $(this)
         .animate(transforms[i], duration)
    }).end()
}(jQuery))
.border {
    margin: 20px 50px;
    width: 50px;
    height: 50px;
    border-width: 50px;
    border-style: solid;
    border-top-color: green;
    border-right-color: yellow;
    border-bottom-color: red;
    border-left-color: blue;
    cursor: pointer
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script src="https://code.jquery.com/color/jquery.color-2.1.2.min.js"></script>
Click it!<br>
<div class="border"></div>
function shuffleArray(array) {
    for (var i = array.length - 1; i > 0; i--) {
        var j = Math.floor(Math.random() * (i + 1));
        var temp = array[i];
        array[i] = array[j];
        array[j] = temp;
    return array;
transforms = [
         {'border-left-width'   :'30', 'margin-left': '70'},
         {'border-bottom-width' :'80'},
         {'border-right-width'  :'30'},
         {'border-top-width'    :'0', 'margin-top': '70'},
         {'width'               :'0'},
         {'height'              :'0'},
         {'borderLeftColor'     :'transparent'},
         {'borderRightColor'    :'transparent'}
transforms = shuffleArray(transforms)
$('#a').click(function() {$('.border').trigger("click");});
(function($) {
    var duration = 1000
    $('.border').click(function() {
		  for ( var i=0; i < transforms.length; i++ ) {
        $(this)
         .animate(transforms[i], duration)
    }).end()
}(jQuery))
.border {
    margin: 50px;
    width: 50px;
    height: 50px;
    border-width: 50px;
    border-style: solid;
    border-top-color: green;
    border-right-color: yellow;
    border-bottom-color: red;
    border-left-color: blue;
    cursor: pointer
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script src="https://code.jquery.com/color/jquery.color-2.1.2.min.js"></script>
Click it!<br>
<div class="border"></div>
$('#a').click(function() {$('.border').trigger("click");});
(function($) {
    var duration = 1000
    $('.border').click(function() {
        $(this)
         .animate({'border-top-width': 0            ,
         					 'border-left-width': 30          ,
         					 'border-right-width': 30         ,
         					 'border-bottom-width': 80        ,
         					 'width': 0                       ,
         					 'height': 0                      ,
                   'margin-left': 100,
                   'margin-top': 150,
         					 'borderTopColor': 'transparent',
         					 'borderRightColor': 'transparent',
         					 'borderLeftColor':  'transparent'}, duration)
    }).end()
}(jQuery))
.border {
    margin: 50px;
    width: 50px;
    height: 50px;
    border-width: 50px;
    border-style: solid;
    border-top-color: green;
    border-right-color: yellow;
    border-bottom-color: red;
    border-left-color: blue;
    cursor: pointer
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script src="https://code.jquery.com/color/jquery.color-2.1.2.min.js"></script>
Click it!<br>
<div class="border"></div>
Now Edit the CSS step-by-step, so you will get clear idea what is happening around




    

STEP 1: 
JSfiddle Link:
 #triangle {
        background: purple;
        width :150px;
        height:150PX;
        border-left: 50px solid black ;
        border-right: 50px solid black;
        border-bottom: 50px solid black;
        border-top: 50px solid black;
This is a simple div. With a very simple CSS. So a layman can understand. Div has dimensions 150 x 150 pixels with the border 50 pixels. The image is attached:
STEP 2: JSfiddle Link:
#triangle {
    background: purple;
    width :150px;
    height:150PX;
    border-left: 50px solid yellow ;
    border-right: 50px solid green;
    border-bottom: 50px solid red;
    border-top: 50px solid blue;
Now I just changed the border-color of all 4 sides. The image is attached.
STEP:3 JSfiddle Link:
#triangle {
    background: purple;
    width :0;
    height:0;
    border-left: 50px solid yellow ;
    border-right: 50px solid green;
    border-bottom: 50px solid red;
    border-top: 50px solid blue;
Now I just changed the height & width of div from 150 pixels to zero. The image is attached 
STEP 4: JSfiddle:
#triangle {
    background: purple;
    width :0px;
    height:0px;
    border-left: 50px solid transparent;
    border-right: 50px solid transparent;
    border-bottom: 50px solid red;
    border-top: 50px solid transparent;
Now I have made all the borders transparent apart from the bottom border. The image is attached below.
STEP 5: JSfiddle Link:
#triangle {
    background: white;
    width :0px;
    height:0px;
    border-left: 50px solid transparent;
    border-right: 50px solid transparent;
    border-bottom: 50px solid red;
    border-top: 50px solid transparent;
Now I just changed the background color to white. The image is attached.
Hence we got the triangle we needed.
And now something completely different...
Instead of using css tricks don't forget about solutions as simple as html entities:
&#9650;
Result:
See: What are the HTML entities for up and down triangles?
                I don't think "beaten" is an appropriate word here. The solution relies on the font metrics, so the precise positioning is rather dubious, not to mention that you have no control over the shape.
– user776686
                May 19, 2017 at 13:40
                Yes, the result is not very much controllable, but this is usually the best solution when you want to use simple icon for drop-downs etc. I would place it into ::before's or ::after's content.
– jiwopene
                Jul 22, 2022 at 12:06
    border-bottom:15px solid #000;
    border-left:10px solid transparent;
    border-right:10px solid transparent;
    width:0;
    height:0;
This is what we are given:
Why it came out in this shape? The below diagram explains the dimensions, note that 15px was used for the bottom border and 10px was used for left and right.
It's pretty easy to make a right-angle triangle also by removing the right border.
  border-left: 50px solid transparent;
  border-right: 50px solid transparent;
  border-bottom: 100px solid red;
  margin:20px auto;
.triangle-down {
  border-bottom:none;
  border-top: 100px solid red;
.triangle-left {
  border-left:none;
  border-right: 100px solid red;
  border-bottom: 50px solid transparent;
  border-top: 50px solid transparent;
.triangle-right {
  border-right:none;
  border-left: 100px solid red;
  border-bottom: 50px solid transparent;
  border-top: 50px solid transparent;
.triangle-after:after {
  width: 0;
  height: 0;
  border-left: 5px solid transparent;
  border-right: 5px solid transparent;
  border-bottom: 5px solid red;
  margin:0 5px;
  content:"";
  display:inline-block;
.triangle-after-right:after {
  border-right:none;
  border-left: 5px solid blue;
  border-bottom: 5px solid transparent;
  border-top: 5px solid transparent;
.triangle-before:before {
  width: 0;
  height: 0;
  border-left: 5px solid transparent;
  border-right: 5px solid transparent;
  border-bottom: 5px solid blue;
  margin:0 5px;
  content:"";
  display:inline-block;
.triangle-before-left:before {
  border-left:none;
  border-right: 5px solid blue;
  border-bottom: 5px solid transparent;
  border-top: 5px solid transparent;
<div class="triangle"></div>
<div class="triangle triangle-down"></div>
<div class="triangle triangle-left"></div>
<div class="triangle triangle-right"></div>
<a class="triangle-before triangle-before-left" href="#">Back</a>
<a class="triangle-after triangle-after-right" href="#">Next</a>
  
clip-path in a nutshell
  
Clipping, with the clip-path property, is akin to cutting a shape (like a circle or a pentagon) from a rectangular piece of paper. The property belongs to the “CSS Masking Module Level 1” specification. The spec states, “CSS masking provides two means for partially or fully hiding portions of visual elements: masking and clipping”.
  
Extract from Smashing Magazine
div {
  -webkit-clip-path: polygon(50% 0%, 0% 100%, 100% 100%);
  clip-path: polygon(50% 0%, 0% 100%, 100% 100%);
  background: red;
  width: 100px;
  height: 100px;
<div></div>
Resources
Here are some useful resources and material to help better understand clip-path and also start creating your own.
Clippy - A clip-path generator
The W3C Candidate Recommendation File
MDN clip-path documentation
clip-path Browser Support 
                in 2023 this is a great answer. it's more intuitive than the border trick and much more flexible. it is disappointing to see it so far down below multiple nearly-identical answers discussing borders!
– charcole
                Apr 14 at 15:40
    height: 100px;
/* linear gradient */
    background: -moz-linear-gradient(-45deg,  rgba(255,0,0,0) 0%, rgba(255,0,0,0) 50%, rgba(255,0,0,1) 50%, rgba(255,0,0,1) 100%);
 /* FF3.6+ */
    background: -webkit-gradient(linear, left top, right bottom, color-stop(0%,rgba(255,0,0,0)), color-stop(50%,rgba(255,0,0,0)), color-stop(50%,rgba(255,0,0,1)), color-stop(100%,rgba(255,0,0,1)));
 /* Chrome,Safari4+ */
    background: -webkit-linear-gradient(-45deg,  rgba(255,0,0,0) 0%,rgba(255,0,0,0) 50%,rgba(255,0,0,1) 50%,rgba(255,0,0,1) 100%);
 /* Chrome10+,Safari5.1+ */
    background: -o-linear-gradient(-45deg,  rgba(255,0,0,0) 0%,rgba(255,0,0,0) 50%,rgba(255,0,0,1) 50%,rgba(255,0,0,1) 100%);
 /* Opera 11.10+ */
    background: -ms-linear-gradient(-45deg,  rgba(255,0,0,0) 0%,rgba(255,0,0,0) 50%,rgba(255,0,0,1) 50%,rgba(255,0,0,1) 100%);
 /* IE10+ */
    background: linear-gradient(135deg,  rgba(255,0,0,0) 0%,rgba(255,0,0,0) 50%,rgba(255,0,0,1) 50%,rgba(255,0,0,1) 100%);
 /* W3C */;
<div class="triangle"></div>
OK, this triangle will get created because of the way that borders of the elements work together in HTML and CSS... 
As we usually use 1 or 2px borders, we never notice that borders make a 45° angles to each others with the same width and if the width changes, the angle degree get changed as well, run the CSS code I created below:
.triangle {
  width: 100px;
  height: 100px;
  border-left: 50px solid black;
  border-right: 50px solid black;
  border-bottom: 100px solid red;
<div class="triangle">
  border-left: 50px solid black;
  border-right: 50px solid black;
  border-bottom: 100px solid red;
<div class="triangle">
  border-left: 50px solid transparent;
  border-right: 50px solid transparent;
  border-bottom: 100px solid red;
<div class="triangle"></div>
This is an old question, but I think will worth it to share how to create an arrow using this triangle technique.
Step 1:
Lets create 2 triangles, for the second one we will use the :after pseudo class and position it just below the other:
.arrow{
    width: 0;
    height: 0;
    border-radius: 50px;
    display: inline-block;
    position: relative;
    .arrow:after{
        content: "";
        width: 0;
        height: 0;
        position: absolute;
.arrow-up{
     border-left: 50px solid transparent;
     border-right: 50px solid transparent;
     border-bottom: 50px solid #333;
    .arrow-up:after{
         top: 5px;
         border-left: 50px solid transparent;
         border-right: 50px solid transparent;
         border-bottom: 50px solid #ccc;
         right: -50px;
<div class="arrow arrow-up"> </div>
Step 2
Now we just have to set the predominant border color of the second triangle to the same color of the background:
.arrow{
    width: 0;
    height: 0;
    border-radius: 50px;
    display: inline-block;
    position: relative;
    .arrow:after{
        content: "";
        width: 0;
        height: 0;
        position: absolute;
.arrow-up{
     border-left: 50px solid transparent;
     border-right: 50px solid transparent;
     border-bottom: 50px solid #333;
    .arrow-up:after{
         top: 5px;
         border-left: 50px solid transparent;
         border-right: 50px solid transparent;
         border-bottom: 50px solid #fff;
         right: -50px;
<div class="arrow arrow-up"> </div>
Fiddle with all the arrows:

http://jsfiddle.net/tomsarduy/r0zksgeu/
Almost all the answers focus on the triangle built using border so I am going to elaborate the linear-gradient method (as started in the answer of @lima_fil).
Using a degree value like 45° will force us to respect a specific ratio of height/width in order to obtain the triangle we want and this won't be responsive:
.tri {
  width:100px;
  height:100px;
  background:linear-gradient(45deg, transparent 49.5%,red 50%);
  /*To illustrate*/
  border:1px solid;
Good one
<div class="tri"></div>
bad one
<div class="tri" style="width:150px"></div>
bad one
<div class="tri" style="height:30px"></div>
Instead of doing this we should consider predefined values of direction like to bottom, to top, etc. In this case we can obtain any kind of triangle shape while keeping it responsive.
1) Rectangle triangle
To obtain such triangle we need one linear-gradient and a diagonal direction like to bottom right, to top left, to bottom left, etc
.tri-1,.tri-2 {
  display:inline-block;
  width:100px;
  height:100px;
  background:linear-gradient(to bottom left, transparent 49.5%,red 50%);
  border:1px solid;
  animation:change 2s linear infinite alternate;
.tri-2 {
  background:linear-gradient(to top right, transparent 49.5%,red 50%);
  border:none;
@keyframes change {
  from {
    width:100px;
    height:100px;
    height:50px;
    width:180px;
<div class="tri-1"></div>
<div class="tri-2"></div>
  height:100px;
  background-image:
  linear-gradient(to bottom right, transparent 49.5%,red 50%),
  linear-gradient(to bottom left,  transparent 49.5%,red 50%);
  background-size:50.3% 100%; /* I use a value slightly bigger than 50% to avoid having a small gap between both gradient*/
  background-position:left,right;
  background-repeat:no-repeat;
  animation:change 2s linear infinite alternate;
@keyframes change {
  from {
    width:100px;
    height:100px;
    height:50px;
    width:180px;
<div class="tri"></div>
3) equilateral triangle
This one is a bit tricky to handle as we need to keep a relation between the height and width of the gradient. We will have the same triangle as above but we will make the calculation more complex in order to transform the isosceles triangle to an equilateral one.
To make it easy, we will consider that the width of our div is known and the height is big enough to be able to draw our triangle inside (height >= width).
We have our two gradient g1 and g2, the blue line is the width of the div  w and each gradient will have 50% of it (w/2) and each side of the triangle sould be equal to w. The green line is the height of both gradient hg and we can easily obtain the formula below:
(w/2)² + hg² = w² ---> hg = (sqrt(3)/2) * w ---> hg = 0.866 * w
We can  rely on calc() in order to do our calculation and to obtain the needed result:
.tri {
  --w:100px;
  width:var(--w);
  height:100px;
  display:inline-block;
  background-image:
  linear-gradient(to bottom right, transparent 49.5%,red 50%),
  linear-gradient(to bottom left,  transparent 49.5%,red 50%);
  background-size:calc(var(--w)/2 + 0.5px)  calc(0.866 * var(--w));
  background-position:
    left bottom,right bottom;
  background-repeat:no-repeat;
<div class="tri"></div>
<div class="tri" style="--w:80px"></div>
<div class="tri" style="--w:50px"></div>
  display:inline-block;
  background:
   linear-gradient(to bottom right, transparent 49.8%,red 50%) left,
   linear-gradient(to bottom left,  transparent 49.8%,red 50%) right;
  background-size:50.2% 100%;
  background-repeat:no-repeat;
<div class="tri"></div>
<div class="tri" style="--w:80px"></div>
<div class="tri" style="--w:50px"></div>
  display:inline-block;
  background-image:
  linear-gradient(to bottom right, transparent 50%,red 0),
  linear-gradient(to bottom left, transparent 50%,red 0);
  background-size:20% 60%,80% 60%;
  background-position:
    left bottom,right bottom;
  background-repeat:no-repeat;
<div class="tri-1"></div>
Let's define hg1 and hg2 as the height of our gradient (both are equal to the red line) then wg1 and wg2 as the width of our gradient (wg1 + wg2 = a). I will not going to detail the calculation but at then end we will have:
wg2 = (a²+c²-b²)/(2a)
wg1 = a - wg2
hg1 = hg2 = sqrt(b² - wg1²) = sqrt(c² - wg2²)
Now we have reached the limit of CSS as even with calc() we won't be able to implement this so we simply need to gather the final result manually and use them as fixed size:
.tri {
  --wg1: 20px; 
  --wg2: 60px;
  --hg:30px; 
  width:calc(var(--wg1) + var(--wg2));
  height:100px;
  display:inline-block;
  background-image:
  linear-gradient(to bottom right, transparent 49.5%,red 50%),
  linear-gradient(to bottom left,  transparent 49.5%,red 50%);
  background-size:var(--wg1) var(--hg),var(--wg2) var(--hg);
  background-position:
    left bottom,right bottom;
  background-repeat:no-repeat;
<div class="tri" ></div>
<div class="tri" style="--wg1:80px;--wg2:60px;--hg:100px;" ></div>
  display:inline-block;
  background-image:
  linear-gradient(to bottom right, transparent 49%,red 50%),
  linear-gradient(to bottom left,  transparent 49%,red 50%);
  background-size:var(--wg1) var(--hg),var(--wg2) var(--hg);
  background-position:
    left bottom,right bottom;
  background-repeat:no-repeat;
<div class="tri" ></div>
<div class="tri" style="transform:skewY(25deg)"></div>
<div class="tri" style="--wg1:80px;--wg2:60px;--hg:100px;" ></div>
<div class="tri" style="--wg1:80px;--wg2:60px;--hg:100px;transform:rotate(20deg)" ></div>
<svg viewBox="0 0 100 100"><polygon points="0,100 0,0 100,100" /></svg>
<svg viewBox="0 0 100 100"><polygon points="0,100 50,0 100,100" /></svg>
<svg viewBox="0 0 100 100"><polygon points="0,100 50,23 100,100" /></svg>
<svg viewBox="0 0 100 100"><polygon points="20,60 50,43 80,100" /></svg>
SASS (SCSS) triangle mixin
I wrote this to make it easier (and DRY) to automatically generate a CSS triangle:
// Triangle helper mixin (by Yair Even-Or)
// @param {Direction} $direction - either `top`, `right`, `bottom` or `left`
// @param {Color} $color [currentcolor] - Triangle color
// @param {Length} $size [1em] - Triangle size
@mixin triangle($direction, $color: currentcolor, $size: 1em) {
  $size: $size/2;
  $transparent: rgba($color, 0);
  $opposite: (top:bottom, right:left, left:right, bottom:top);
  content: '';
  display: inline-block;
  width: 0;
  height: 0;
  border: $size solid $transparent;
  border-#{map-get($opposite, $direction)}-color: $color;
  margin-#{$direction}: -$size;
use-case example:
span {
  @include triangle(bottom, red, 10px);
Playground page
Important note:
 if the triangle seems pixelated in some browsers, try one of the methods described here.
  window.requestAnimationFrame(() => {
    triangle.style.borderWidth = sizes.map(size => `${ size }px`).join(' ');
    if (isNaN(index)) {
      triangle.style[target.id] = `${ target.value }px`;
document.querySelectorAll('input').forEach(input => {
  input.oninput = update;
update({});
body {
  margin: 0;
  min-height: 100vh;
  display: flex;
  justify-content: center;
  align-items: center;
  overflow: hidden;
#triangle {
    border-style: solid;
    border-color: yellow magenta blue black;
    background: cyan;
    ;
    width: 0px;
#controls {
  position: fixed;
  bottom: 0;
  left: 0;
  right: 0;
  background: white;
  display: flex;
  box-shadow: 0 0 32px rgba(0, 0, 0, .125);
#controls > div {
  position: relative;
  width: 25%;
  padding: 8px;
  box-sizing: border-box;
  display: flex;
input {
  margin: 0;
  width: 100%;
  position: relative;
<div id="triangle" style="border-width: 32px 32px 32px 32px;"></div>
<div id="controls">
  <div><input type="range" min="0" max="128" value="32" id="0" /></div>
  <div><input type="range" min="0" max="128" value="32" id="1" /></div>
  <div><input type="range" min="0" max="128" value="32" id="2" /></div>
  <div><input type="range" min="0" max="128" value="32" id="3" /></div>
  <div><input type="range" min="0" max="128" value="0" id="width" /></div>
  <div><input type="range" min="0" max="128" value="0" id="height" /></div>
    border-width: 25px;
    border-style: solid;
    border-color: transparent transparent transparent #000;
    width: 0;
    height: 0;
    z-index: 10;
    -webkit-transition: visibility 50ms ease-in-out,opacity 50ms ease-in-out;
    transition: visibility 50ms ease-in-out,opacity 50ms ease-in-out;
    bottom: 21px;
.container {
    float: left;
    margin-top: 100px;
    position: relative;
    width: 150px;
    height: 80px;
    background-color: #000;
.containerRed {
    float: left;
    margin-top: 100px;
    position: relative;
    width: 100px;
    height: 80px;
    background-color: red;
https://jsfiddle.net/qdhvdb17/
Others have already explained this well. Let me give you an animation which will explain this quickly: http://codepen.io/chriscoyier/pen/lotjh
Here is some code for you to play with and learn the concepts.
HTML:
    <div id="border-demo">
  </body>
</html>
/*border-width is border thickness*/
#border-demo {
    background: gray;
    border-color: yellow blue red green;/*top right bottom left*/
    border-style: solid;
    border-width: 25px 25px 25px 25px;/*top right bottom left*/
    height: 50px;
    width: 50px;
Play with this and see what happens. Set height and width to zero. Then remove top border and make left and right transparent, or just look at the code below to make a css triangle:
#border-demo {
    border-left: 50px solid transparent;
    border-right: 50px solid transparent;
    border-bottom: 100px solid blue;
I know this is an old one, but I'd like to add to this discussion that There are at least 5 different methods for creating a triangle using HTML & CSS alone.
Using borders 
Using linear-gradient 
Using conic-gradient 
Using
transform and overflow 
Using clip-path
I think that all have been covered here except for method 3, using the conic-gradient, so I will share it here:
.triangle{
        width: 40px;
        height: 40px;
        background: conic-gradient(at 50% 50%,transparent 135deg,green 0,green 225deg, transparent 0);
<div class="triangle"></div>
use clip-path: polygon(50% 0%, 100% 100%, 0% 100%); for creating easy to Triangle
<div class="triangle"></div>
.triangle{width:200px; height:200px;background:#000;clip-path: polygon(50% 0%, 100% 100%, 0% 100%);}
After reading through the other answers here I see there are great explanations as to why the CSS triangle works the way it does. I would consider it to be somewhat of a trick, versus something that you could apply generically.
For something that's easier to read and maintain, I would recommend you define your geometry in SVG.
Then you can convert that SVG using data uri by adding the data:image/svg+xml, prefix. As a data uri, it can now be used as a background-image in a CSS. Because the SVG is in clear text, you can readily make updates to the geometry, stroke, and fill color.
div.tri {
    width: 100px;
    height: 100px;
    display: inline-block;
    background-image: url('data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32"><path  fill="red" d="M31.345 29H1.655L16.5 1.96z"/></svg>');
    <div class="tri"></div>
    <div class="tri"></div>
    <div class="tri"></div>
    border-color: transparent transparent red transparent;
    border-style: solid;
    border-width: 0px 200px 200px 200px;
    ;
    width: 0px;
<div class="triangle"></div>
clip-path has the best result for me - works great for divs/containers with and without fixed dimensions:
.triangleContainer{
    position: relative;
    width: 500px;
    height: 500px;
.triangleContainer::before{
    content: "";        
    position: absolute;
    background:blue;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    clip-path: polygon(50% 0, 0 100%, 100% 100%);
                not sure what is wrong with my suggestion. I just used it on another project and works a treat - happy to make a triangle on request with a variation on this technique for anyone interested!
– LaZza
                Jul 16, 2021 at 14:12
                The problem was that the OP wanted to know how it works, what is the reason doing this or that results in a triangle? not how to do it.
– user13944038
                Jan 12, 2022 at 17:11