bootstrap/scss/mixins/_grid-framework.scss

// Framework grid generation
//
// Used only by Bootstrap to generate the correct number of grid classes given
// any value of `$grid-columns`.

@mixin make-grid-columns($columns: $grid-columns, $gutter: $grid-gutter-width, $breakpoints: $grid-breakpoints) {

  // Common properties for all breakpoints
  %grid-column {
    position: relative;
    // Prevent columns from collapsing when empty
    min-height: 1px;
    // Inner gutter via padding
    padding-left: ($gutter / 2);
    padding-right: ($gutter / 2);

    @if $enable-flex {
      width: 100%;
    }
  }

  $breakpoint-counter: 0;
  @each $breakpoint in map-keys($breakpoints) {
    $breakpoint-counter: ($breakpoint-counter + 1);

    @for $i from 1 through $columns {
      .col-#{$breakpoint}-#{$i} {
        @extend %grid-column;
      }
    }

    @include media-breakpoint-up($breakpoint, $breakpoints) {
      @for $i from 1 through $columns {
        .col-#{$breakpoint}-#{$i} {
          @include make-col($i, $columns, $gutter);
        }
      }

      @each $modifier in (pull, push) {
        @for $i from 0 through $columns {
          .#{$modifier}-#{$breakpoint}-#{$i} {
            @include make-col-modifier($modifier, $i, $columns)
          }
        }
      }

      // `$columns - 1` because offsetting by the width of an entire row isn't possible
      @for $i from 0 through ($columns - 1) {
        @if $breakpoint-counter != 1 or $i != 0 { // Avoid emitting useless .offset-xs-0
          .offset-#{$breakpoint}-#{$i} {
            @include make-col-modifier(offset, $i, $columns)
          }
        }
      }
    }
  }
}