twbs--bootstrap/scss/utilities/_spacing.scss

// Width and height

.w-25 { width: 25% !important; }
.w-50 { width: 50% !important; }
.w-75 { width: 75% !important; }
.w-100 { width: 100% !important; }

.h-25 { height: 25% !important; }
.h-50 { height: 50% !important; }
.h-75 { height: 75% !important; }
.h-100 { height: 100% !important; }

.mw-100 { max-width: 100% !important; }
.mh-100 { max-height: 100% !important; }

// Margin and Padding

@each $breakpoint in map-keys($grid-breakpoints) {
  @each $prop, $abbrev in (margin: m, padding: p) {
    @each $size, $lengths in $spacers {
      $length-x:   map-get($lengths, x);
      $length-y:   map-get($lengths, y);

      @include media-breakpoint-up($breakpoint) {
        $infix: breakpoint-infix($breakpoint, $grid-breakpoints);

        .#{$abbrev}#{$infix}-#{$size}  { #{$prop}:        $length-y $length-x !important; }
        .#{$abbrev}t#{$infix}-#{$size} { #{$prop}-top:    $length-y !important; }
        .#{$abbrev}r#{$infix}-#{$size} { #{$prop}-right:  $length-x !important; }
        .#{$abbrev}b#{$infix}-#{$size} { #{$prop}-bottom: $length-y !important; }
        .#{$abbrev}l#{$infix}-#{$size} { #{$prop}-left:   $length-x !important; }
        .#{$abbrev}x#{$infix}-#{$size} {
          #{$prop}-right:  $length-x !important;
          #{$prop}-left:   $length-x !important;
        }
        .#{$abbrev}y#{$infix}-#{$size} {
          #{$prop}-top:    $length-y !important;
          #{$prop}-bottom: $length-y !important;
        }
      }
    }
  }

  // Some special margin utils
  @include media-breakpoint-up($breakpoint) {
    $infix: breakpoint-infix($breakpoint, $grid-breakpoints);
    
    .m#{$infix}-auto { margin: auto !important; }
    .mx#{$infix}-auto {
      margin-right: auto !important;
      margin-left:  auto !important;
    }
    .my#{$infix}-auto {
      margin-top: auto !important;
      margin-bottom:  auto !important;
    }
    .mt#{$infix}-auto { margin-top: auto !important; }
    .mr#{$infix}-auto { margin-right: auto !important; }
    .mb#{$infix}-auto { margin-bottom: auto !important; }
    .ml#{$infix}-auto { margin-left: auto !important; }
  }
}

// Positioning

.pos-f-t {
  position: fixed;
  top: 0;
  right: 0;
  left: 0;
  z-index: $zindex-navbar-fixed;
}