foundation-sites/scss/prototype/_box.scss

// Foundation for Sites by ZURB
// foundation.zurb.com
// Licensed under MIT Open Source

////
/// @group prototype-box
////

/// Box Mixin: Easily create a square, rectangle or a circle
/// @param {Number} $width[] Width of the box
/// @param {Number} $height[$width] Height of the box, defaults to `$width` to easily make a square
/// @param {Boolean} $circle[false] Makes the box a circle, by default `false`.
@mixin box(
  $width, 
  $height: $width, 
  $circle: false
) {
  width: $width;
  height: $height;
  @if $circle {
    border-radius: 50% !important;
  }
}