twbs--bootstrap/scss/_grid.scss

// Row
//
// Rows contain your columns.

@if $enable-grid-classes {
  .row {
    @include make-row();

    > * {
      @include make-col-ready();
    }
  }
}

// Gutters
//
// Make use of `.g-*`, `.gx-*` or `.gy-*` utilities to change spacing between the columns.

@if $enable-grid-classes {
  @each $breakpoint in map-keys($grid-breakpoints) {
    $infix: breakpoint-infix($breakpoint, $grid-breakpoints);

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

      @each $key, $value in $gutters {
        .g#{$infix}-#{$key},
        .gx#{$infix}-#{$key} {
          margin-right: -$value / 2;
          margin-left: -$value / 2;

          > * {
            padding-right: $value / 2;
            padding-left: $value / 2;
          }
        }

        .g#{$infix}-#{$key},
        .gy#{$infix}-#{$key} {
          margin-top: -$value;

          > * {
            margin-top: $value;
          }
        }
      }
    }
  }
}

// Columns
//
// Common styles for small and large grid columns

@if $enable-grid-classes {
  @include make-grid-columns();
}