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Bootstrap class: .dropdown-divider

<div class="dropdown open">
  <button class="btn btn-secondary dropdown-toggle"
          type="button" id="dropdownMenu5" data-toggle="dropdown"
          aria-haspopup="true" aria-expanded="false">
    Dropdown
  </button>
  <div class="dropdown-menu">
    <a class="dropdown-item" href="#">Separated link</a>
    <div class="dropdown-divider"></div>
    <a class="dropdown-item" href="#">Action</a>
    <a class="dropdown-item" href="#">Another action</a>
  </div>
</div>

Preview

Sass source

/* _dropdown.scss:104 */
.dropdown-divider {
  @include nav-divider($dropdown-divider-bg);
}

Uses variables

/* _dropdown.scss:104 */
$dropdown-divider-bg