twbs--bootstrap/scss/_grid.scss

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

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

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


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

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