twbs--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);
  }
  @each $breakpoint in map-keys($breakpoints) {
    @for $i from 1 through $columns {
      .col-#{$breakpoint}-#{$i} {
        @extend %grid-column;
      }
    }
    @include media-breakpoint-up($breakpoint) {
      // Work around cross-media @extend (https://github.com/sass/sass/issues/1050)
      %grid-column-float-#{$breakpoint} {
        float: left;
      }
      @for $i from 1 through $columns {
        .col-#{$breakpoint}-#{$i} {
          @if not $enable-flex {
            @extend %grid-column-float-#{$breakpoint};
          }
          @include make-col-span($i, $columns);
        }
      }
      @each $modifier in (pull, push, offset) {
        @for $i from 0 through $columns {
          .col-#{$breakpoint}-#{$modifier}-#{$i} {
            @include make-col-modifier($modifier, $i, $columns)
          }
        }
      }
    }
  }
}