bootstrap4.css
.dropdown-toggle::after {
  display: inline-block;
  margin-left: 0.255em;
  vertical-align: 0.255em;
  content: "";
  border-top: 0.3em solid;
  border-right: 0.3em solid transparent;
  border-bottom: 0;
  border-left: 0.3em solid transparent;
}
bootstrap4.css
.dropup .dropdown-toggle::after {
  display: inline-block;
  margin-left: 0.255em;
  vertical-align: 0.255em;
  content: "";
  border-top: 0;
  border-right: 0.3em solid transparent;
  border-bottom: 0.3em solid;
  border-left: 0.3em solid transparent;
}
bootstrap4.css
.dropright .dropdown-toggle::after {
  display: inline-block;
  margin-left: 0.255em;
  vertical-align: 0.255em;
  content: "";
  border-top: 0.3em solid transparent;
  border-right: 0;
  border-bottom: 0.3em solid transparent;
  border-left: 0.3em solid;
}
bootstrap4.css
.custom-file-label::after {
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  z-index: 3;
  display: block;
  height: calc(1.5em + 0.75rem);
  padding: 0.375rem 0.75rem;
  line-height: 1.5;
  color: #495057;
  content: "Browse";
  background-color: #e9ecef;
  border-left: inherit;
  border-radius: 0 0.25rem 0.25rem 0;
}
bootstrap4.css
@media (min-width: 576px) {
  .card-group {
    -ms-flex-flow: row wrap;
    flex-flow: row wrap;
  }
  .card-group > .card {
    -ms-flex: 1 0 0%;
    flex: 1 0 0%;
    margin-bottom: 0;
  }
  .card-group > .card + .card {
    margin-left: 0;
    border-left: 0;
  }
  .card-group > .card:not(:last-child) {
    border-top-right-radius: 0;
    border-bottom-right-radius: 0;
  }
  .card-group > .card:not(:last-child) .card-img-top,
  .card-group > .card:not(:last-child) .card-header {
    border-top-right-radius: 0;
  }
  .card-group > .card:not(:last-child) .card-img-bottom,
  .card-group > .card:not(:last-child) .card-footer {
    border-bottom-right-radius: 0;
  }
  .card-group > .card:not(:first-child) {
    border-top-left-radius: 0;
    border-bottom-left-radius: 0;
  }
  .card-group >
bootstrap4.css
.list-group-flush .list-group-item {
  border-right: 0;
  border-left: 0;
  border-radius: 0;
}
bootstrap4.css
.bs-tooltip-left .arrow::before, .bs-tooltip-auto[x-placement^="left"] .arrow::before {
  left: 0;
  border-width: 0.4rem 0 0.4rem 0.4rem;
  border-left-color: #000;
}
bootstrap4.css
.bs-popover-left > .arrow::before, .bs-popover-auto[x-placement^="left"] > .arrow::before {
  right: 0;
  border-width: 0.5rem 0 0.5rem 0.5rem;
  border-left-color: rgba(0, 0, 0, 0.25);
}
bootstrap4.css
.bs-popover-left > .arrow::after, .bs-popover-auto[x-placement^="left"] > .arrow::after {
  right: 1px;
  border-width: 0.5rem 0 0.5rem 0.5rem;
  border-left-color: #fff;
}
bootstrap4.css
.border-left {
  border-left: 1px solid #dee2e6 !important;
}
bootstrap4.css
.border-left-0 {
  border-left: 0 !important;
}
■外枠:Border:ユーティリティ:ボーダーを追加するクラス !important

border border-top border-right border-bottom border-left
<span class="border">border</span><!-- 全辺ボーダー -->
<span class="border-top">border-top</span><!-- 上ボーダー -->
<span class="border-right">border-right</span><!-- 右ボーダー -->
<span class="border-bottom">border-bottom</span><!-- 下ボーダー -->
<span class="border-left">border-left</span><!-- 左ボーダー -->
■外枠:Border:ユーティリティ:ボーダーを削除するクラス

border-0 border-top-0 border-right-0 border-bottom-0 border-left-0
<span class="border border-0">border-0</span><!-- 全ボーダー削除 -->
<span class="border border-top-0">border-top-0</span><!-- 上ボーダー削除 -->
<span class="border border-right-0">border-right-0</span><!-- 右ボーダー削除 -->
<span class="border border-bottom-0">border-bottom-0</span><!-- 下ボーダー削除 -->
<span class="border border-left-0">border-left-0</span><!-- 左ボーダー削除 -->