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`.

// Common properties for all breakpoints
@mixin make-grid-columns($columns: $grid-columns, $breakpoints: $grid-breakpoints) {
  %grid-column {
    position: relative;
    // Prevent columns from collapsing when empty
    min-height: 1px;
    // Inner gutter via padding
    padding-left: ($grid-gutter-width / 2);
    padding-right: ($grid-gutter-width / 2);
  }
  @for $i from 1 through $columns {
    @each $breakpoint in $breakpoints {
      .col-#{$breakpoint}-#{$i} {
        @extend %grid-column;
      }
    }
  }
}

// Breakpoint-specific properties
@mixin make-grid($breakpoint, $columns: $grid-columns) {
  // Work around cross-media @extend (https://github.com/sass/sass/issues/1050)
  %grid-column-float-#{$breakpoint} {
    float: left;
  }
  @for $i from 1 through $columns {
    .col-#{$breakpoint}-#{$i} {
      @extend %grid-column-float-#{$breakpoint};
      @include grid-column-width($i, $columns);
    }
  }
  @each $modifier in (pull, push, offset) {
    @for $i from 0 through $columns {
      .col-#{$breakpoint}-#{$modifier}-#{$i} {
        @include grid-column-modifier($modifier, $i, $columns)
      }
    }
  }
}

@mixin grid-column-width($index, $columns) {
  width: percentage($index / $columns);
}

@mixin grid-column-push($index, $columns) {
  left: if($index > 0, percentage($index / $columns), auto);
}

@mixin grid-column-pull($index, $columns) {
  right: if($index > 0, percentage($index / $columns), auto);
}

@mixin grid-column-offset($index, $columns) {
  margin-left: percentage($index / $columns);
}

// Work around the lack of dynamic mixin @include support (https://github.com/sass/sass/issues/626)
@mixin grid-column-modifier($type, $index, $columns) {
  @if $type == push {
    @include grid-column-push($index, $columns);
  } @else if $type == pull {
    @include grid-column-pull($index, $columns);
  } @else if $type == offset {
    @include grid-column-offset($index, $columns);
  }
}