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`.

%twbs-grid-column {
  position: relative;
  // Prevent columns from collapsing when empty
  min-height: 1px;
  // Inner gutter via padding
  padding-left: ($grid-gutter-width / 2);
  padding-right: ($grid-gutter-width / 2);
}

%twbs-grid-column-float {
  float: left
}

@mixin make-grid-columns($columns: $grid-columns) {
  @for $i from 1 through $columns {
    .col-xs-#{$i}, .col-sm-#{$i}, .col-md-#{$i}, .col-lg-#{$i}, .col-xl-#{$i} {
      @extend %twbs-grid-column;
    }
  }
}

@mixin float-grid-columns($class, $columns: $grid-columns) {
  @for $i from 1 through $columns {
    .col-#{$class}-#{$i} {
      @extend %twbs-grid-column-float;
    }
  }
}

@mixin calc-grid-column($index, $class, $type, $columns: $grid-columns) {
  @if ($type == width) and ($index > 0) {
    .col-#{$class}-#{$index} {
      width: percentage($index / $columns);
    }
  }
  @if $type == push {
    .col-#{$class}-push-#{$index} {
      left: if($index > 0, percentage($index / $columns), auto);
    }
  }
  @if $type == pull {
    .col-#{$class}-pull-#{$index} {
      right: if($index > 0, percentage($index / $columns), auto);
    }
  }
  @if $type == offset {
    .col-#{$class}-offset-#{$index} {
      margin-left: percentage($index / $columns);
    }
  }
}

// Create grid for specific class
@mixin make-grid($class, $columns: $grid-columns) {
  @include float-grid-columns($class);
  @for $i from 0 through $columns {
    @include calc-grid-column($i, $class, width, $columns);
    @include calc-grid-column($i, $class, push, $columns);
    @include calc-grid-column($i, $class, pull, $columns);
    @include calc-grid-column($i, $class, offset, $columns);
  }
}