Font-Awesome/assets/font-awesome/scss/_stacked.scss

// Stacked Icons
// -------------------------

.#{$fa-css-prefix}-stack {
  position: relative;
  display: inline-block;
  width: 2em;
  height: 2em;
  line-height: 2em;
  vertical-align: middle;
}
.#{$fa-css-prefix}-stack-1x, .#{$fa-css-prefix}-stack-2x {
  position: absolute;
  left: 0;
  width: 100%;
  text-align: center;
}
.#{$fa-css-prefix}-stack-1x { line-height: inherit; }
.#{$fa-css-prefix}-stack-2x { font-size: 2em; }
.#{$fa-css-prefix}-inverse { color: $fa-inverse; }