twbs--bootstrap/scss/_grid.scss

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

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

  // Remove the negative margin from default .row, then the horizontal padding
  // from all immediate children columns (to prevent runaway style inheritance).
  .no-gutters {
    margin-right: 0;
    margin-left: 0;

    > .col,
    > [class*="col-"] {
      padding-right: 0;
      padding-left: 0;
    }
  }
}

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

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