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Bootstrap class: .no-gutters

<div class="container">
  <div class="row no-gutters">
    <div class="col-12 col-sm-6 col-md-8">.col-12 .col-sm-6 .col-md-8</div>
    <div class="col-6 col-md-4">.col-6 .col-md-4</div>
  </div>
</div>

Preview

.col-12 .col-sm-6 .col-md-8
.col-6 .col-md-4

Sass source

/* _grid.scss:34 */
.no-gutters {
  margin-right: 0;
  margin-left: 0;

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

Uses variables

/* _grid.scss:34 */