_sass/vendor/bourbon/addons/_triangle.scss

@mixin triangle ($size, $color, $direction) {
  height: 0;
  width: 0;

  $width: nth($size, 1);
  $height: nth($size, length($size));

  $foreground-color: nth($color, 1);
  $background-color: if(length($color) == 2, nth($color, 2), transparent);

  @if ($direction == up) or ($direction == down) or ($direction == right) or ($direction == left) {

    $width: $width / 2;
    $height: if(length($size) > 1, $height, $height/2);

    @if $direction == up {
      border-left: $width solid $background-color;
      border-right: $width solid $background-color;
      border-bottom: $height solid $foreground-color;

    } @else if $direction == right {
      border-top: $width solid $background-color;
      border-bottom: $width solid $background-color;
      border-left: $height solid $foreground-color;

    } @else if $direction == down {
      border-left: $width solid $background-color;
      border-right: $width solid $background-color;
      border-top: $height solid $foreground-color;

    } @else if $direction == left {
      border-top: $width solid $background-color;
      border-bottom: $width solid $background-color;
      border-right: $height solid $foreground-color;
    }
  }

  @else if ($direction == up-right) or ($direction == up-left) {
    border-top: $height solid $foreground-color;

    @if $direction == up-right {
      border-left:  $width solid $background-color;

    } @else if $direction == up-left {
      border-right: $width solid $background-color;
    }
  }

  @else if ($direction == down-right) or ($direction == down-left) {
    border-bottom: $height solid $foreground-color;

    @if $direction == down-right {
      border-left:  $width solid $background-color;

    } @else if $direction == down-left {
      border-right: $width solid $background-color;
    }
  }

  @else if ($direction == inset-up) {
    border-width: $height $width;
    border-style: solid;
    border-color: $background-color $background-color $foreground-color;
  }

  @else if ($direction == inset-down) {
    border-width: $height $width;
    border-style: solid;
    border-color: $foreground-color $background-color $background-color;
  }

  @else if ($direction == inset-right) {
    border-width: $width $height;
    border-style: solid;
    border-color: $background-color $background-color $background-color $foreground-color;
  }

  @else if ($direction == inset-left) {
    border-width: $width $height;
    border-style: solid;
    border-color: $background-color $foreground-color $background-color $background-color;
  }
}