bootstrap4.css
hr {
  margin-top: 1rem;
  margin-bottom: 1rem;
  border: 0;
  border-top: 1px solid rgba(0, 0, 0, 0.1);
}
bootstrap4.css
.table th,
.table td {
  padding: 0.75rem;
  vertical-align: top;
  border-top: 1px solid #dee2e6;
}
bootstrap4.css
.table tbody + tbody {
  border-top: 2px solid #dee2e6;
}
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
.dropleft .dropdown-toggle::before {
  display: inline-block;
  margin-right: 0.255em;
  vertical-align: 0.255em;
  content: "";
  border-top: 0.3em solid transparent;
  border-right: 0.3em solid;
  border-bottom: 0.3em solid transparent;
}
bootstrap4.css
.dropdown-divider {
  height: 0;
  margin: 0.5rem 0;
  overflow: hidden;
  border-top: 1px solid #e9ecef;
}
bootstrap4.css
.btn-group > .btn:not(:last-child):not(.dropdown-toggle),
.btn-group > .btn-group:not(:last-child) > .btn {
  border-top-right-radius: 0;
  border-bottom-right-radius: 0;
}
bootstrap4.css
.btn-group > .btn:not(:first-child),
.btn-group > .btn-group:not(:first-child) > .btn {
  border-top-left-radius: 0;
  border-bottom-left-radius: 0;
}
bootstrap4.css
.btn-group-vertical > .btn:not(:first-child),
.btn-group-vertical > .btn-group:not(:first-child) > .btn {
  border-top-left-radius: 0;
  border-top-right-radius: 0;
}
bootstrap4.css
.input-group > .form-control:not(:last-child),
.input-group > .custom-select:not(:last-child) {
  border-top-right-radius: 0;
  border-bottom-right-radius: 0;
}
bootstrap4.css
.input-group > .form-control:not(:first-child),
.input-group > .custom-select:not(:first-child) {
  border-top-left-radius: 0;
  border-bottom-left-radius: 0;
}
bootstrap4.css
.input-group > .custom-file:not(:last-child) .custom-file-label,
.input-group > .custom-file:not(:last-child) .custom-file-label::after {
  border-top-right-radius: 0;
  border-bottom-right-radius: 0;
}
bootstrap4.css
.input-group > .custom-file:not(:first-child) .custom-file-label {
  border-top-left-radius: 0;
  border-bottom-left-radius: 0;
}
bootstrap4.css
.input-group > .input-group-prepend > .btn,
.input-group > .input-group-prepend > .input-group-text,
.input-group > .input-group-append:not(:last-child) > .btn,
.input-group > .input-group-append:not(:last-child) > .input-group-text,
.input-group > .input-group-append:last-child > .btn:not(:last-child):not(.dropdown-toggle),
.input-group > .input-group-append:last-child > .input-group-text:not(:last-child) {
  border-top-right-radius: 0;
  border-bottom-right-radius: 0;
}
bootstrap4.css
.input-group > .input-group-append > .btn,
.input-group > .input-group-append > .input-group-text,
.input-group > .input-group-prepend:not(:first-child) > .btn,
.input-group > .input-group-prepend:not(:first-child) > .input-group-text,
.input-group > .input-group-prepend:first-child > .btn:not(:first-child),
.input-group > .input-group-prepend:first-child > .input-group-text:not(:first-child) {
  border-top-left-radius: 0;
  border-bottom-left-radius: 0;
}
bootstrap4.css
.nav-tabs .nav-link {
  border: 1px solid transparent;
  border-top-left-radius: 0.25rem;
  border-top-right-radius: 0.25rem;
}
bootstrap4.css
.nav-tabs .dropdown-menu {
  margin-top: -1px;
  border-top-left-radius: 0;
  border-top-right-radius: 0;
}
bootstrap4.css
.card > .list-group:first-child .list-group-item:first-child {
  border-top-left-radius: 0.25rem;
  border-top-right-radius: 0.25rem;
}
bootstrap4.css
.card-header + .list-group .list-group-item:first-child {
  border-top: 0;
}
bootstrap4.css
.card-footer {
  padding: 0.75rem 1.25rem;
  background-color: rgba(0, 0, 0, 0.03);
  border-top: 1px solid rgba(0, 0, 0, 0.125);
}
bootstrap4.css
.card-img-top {
  width: 100%;
  border-top-left-radius: calc(0.25rem - 1px);
  border-top-right-radius: calc(0.25rem - 1px);
}
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
.accordion > .card:last-of-type {
  border-top-left-radius: 0;
  border-top-right-radius: 0;
}
bootstrap4.css
.page-item:first-child .page-link {
  margin-left: 0;
  border-top-left-radius: 0.25rem;
  border-bottom-left-radius: 0.25rem;
}
bootstrap4.css
.page-item:last-child .page-link {
  border-top-right-radius: 0.25rem;
  border-bottom-right-radius: 0.25rem;
}
bootstrap4.css
.pagination-lg .page-item:first-child .page-link {
  border-top-left-radius: 0.3rem;
  border-bottom-left-radius: 0.3rem;
}
bootstrap4.css
.pagination-lg .page-item:last-child .page-link {
  border-top-right-radius: 0.3rem;
  border-bottom-right-radius: 0.3rem;
}
bootstrap4.css
.pagination-sm .page-item:first-child .page-link {
  border-top-left-radius: 0.2rem;
  border-bottom-left-radius: 0.2rem;
}
bootstrap4.css
.pagination-sm .page-item:last-child .page-link {
  border-top-right-radius: 0.2rem;
  border-bottom-right-radius: 0.2rem;
}
bootstrap4.css
.alert-primary hr {
  border-top-color: #9fcdff;
}
bootstrap4.css
.alert-secondary hr {
  border-top-color: #c8cbcf;
}
bootstrap4.css
.alert-success hr {
  border-top-color: #b1dfbb;
}
bootstrap4.css
.alert-info hr {
  border-top-color: #abdde5;
}
bootstrap4.css
.alert-warning hr {
  border-top-color: #ffe8a1;
}
bootstrap4.css
.alert-danger hr {
  border-top-color: #f1b0b7;
}
bootstrap4.css
.alert-light hr {
  border-top-color: #ececf6;
}
bootstrap4.css
.alert-dark hr {
  border-top-color: #b9bbbe;
}
bootstrap4.css
.list-group-item:first-child {
  border-top-left-radius: 0.25rem;
  border-top-right-radius: 0.25rem;
}
bootstrap4.css
.list-group-horizontal .list-group-item:first-child {
  border-top-left-radius: 0.25rem;
  border-bottom-left-radius: 0.25rem;
  border-top-right-radius: 0;
}
bootstrap4.css
.list-group-horizontal .list-group-item:last-child {
  margin-right: 0;
  border-top-right-radius: 0.25rem;
  border-bottom-right-radius: 0.25rem;
  border-bottom-left-radius: 0;
}
bootstrap4.css
@media (min-width: 576px) {
  .list-group-horizontal-sm {
    -ms-flex-direction: row;
    flex-direction: row;
  }
  .list-group-horizontal-sm .list-group-item {
    margin-right: -1px;
    margin-bottom: 0;
  }
  .list-group-horizontal-sm .list-group-item:first-child {
    border-top-left-radius: 0.25rem;
    border-bottom-left-radius: 0.25rem;
    border-top-right-radius: 0;
  }
  .list-group-horizontal-sm .list-group-item:last-child {
    margin-right: 0;
    border-top-right-radius: 0.25rem;
    border-bottom-right-radius: 0.25rem;
    border-bottom-left-radius: 0;
  }
}
bootstrap4.css
@media (min-width: 768px) {
  .list-group-horizontal-md {
    -ms-flex-direction: row;
    flex-direction: row;
  }
  .list-group-horizontal-md .list-group-item {
    margin-right: -1px;
    margin-bottom: 0;
  }
  .list-group-horizontal-md .list-group-item:first-child {
    border-top-left-radius: 0.25rem;
    border-bottom-left-radius: 0.25rem;
    border-top-right-radius: 0;
  }
  .list-group-horizontal-md .list-group-item:last-child {
    margin-right: 0;
    border-top-right-radius: 0.25rem;
    border-bottom-right-radius: 0.25rem;
    border-bottom-left-radius: 0;
  }
}
bootstrap4.css
@media (min-width: 992px) {
  .list-group-horizontal-lg {
    -ms-flex-direction: row;
    flex-direction: row;
  }
  .list-group-horizontal-lg .list-group-item {
    margin-right: -1px;
    margin-bottom: 0;
  }
  .list-group-horizontal-lg .list-group-item:first-child {
    border-top-left-radius: 0.25rem;
    border-bottom-left-radius: 0.25rem;
    border-top-right-radius: 0;
  }
  .list-group-horizontal-lg .list-group-item:last-child {
    margin-right: 0;
    border-top-right-radius: 0.25rem;
    border-bottom-right-radius: 0.25rem;
    border-bottom-left-radius: 0;
  }
}
bootstrap4.css
@media (min-width: 1200px) {
  .list-group-horizontal-xl {
    -ms-flex-direction: row;
    flex-direction: row;
  }
  .list-group-horizontal-xl .list-group-item {
    margin-right: -1px;
    margin-bottom: 0;
  }
  .list-group-horizontal-xl .list-group-item:first-child {
    border-top-left-radius: 0.25rem;
    border-bottom-left-radius: 0.25rem;
    border-top-right-radius: 0;
  }
  .list-group-horizontal-xl .list-group-item:last-child {
    margin-right: 0;
    border-top-right-radius: 0.25rem;
    border-bottom-right-radius: 0.25rem;
    border-bottom-left-radius: 0;
  }
}
bootstrap4.css
.list-group-flush:first-child .list-group-item:first-child {
  border-top: 0;
}
bootstrap4.css
.modal-header {
  display: -ms-flexbox;
  display: flex;
  -ms-flex-align: start;
  align-items: flex-start;
  -ms-flex-pack: justify;
  justify-content: space-between;
  padding: 1rem 1rem;
  border-bottom: 1px solid #dee2e6;
  border-top-left-radius: 0.3rem;
  border-top-right-radius: 0.3rem;
}
bootstrap4.css
.modal-footer {
  display: -ms-flexbox;
  display: flex;
  -ms-flex-align: center;
  align-items: center;
  -ms-flex-pack: end;
  justify-content: flex-end;
  padding: 1rem;
  border-top: 1px solid #dee2e6;
  border-bottom-right-radius: 0.3rem;
  border-bottom-left-radius: 0.3rem;
}
bootstrap4.css
.bs-tooltip-top .arrow::before, .bs-tooltip-auto[x-placement^="top"] .arrow::before {
  top: 0;
  border-width: 0.4rem 0.4rem 0;
  border-top-color: #000;
}
bootstrap4.css
.bs-popover-top > .arrow::before, .bs-popover-auto[x-placement^="top"] > .arrow::before {
  bottom: 0;
  border-width: 0.5rem 0.5rem 0;
  border-top-color: rgba(0, 0, 0, 0.25);
}
bootstrap4.css
.bs-popover-top > .arrow::after, .bs-popover-auto[x-placement^="top"] > .arrow::after {
  bottom: 1px;
  border-width: 0.5rem 0.5rem 0;
  border-top-color: #fff;
}
bootstrap4.css
.popover-header {
  padding: 0.5rem 0.75rem;
  margin-bottom: 0;
  font-size: 1rem;
  background-color: #f7f7f7;
  border-bottom: 1px solid #ebebeb;
  border-top-left-radius: calc(0.3rem - 1px);
  border-top-right-radius: calc(0.3rem - 1px);
}
bootstrap4.css
.carousel-indicators li {
  box-sizing: content-box;
  -ms-flex: 0 1 auto;
  flex: 0 1 auto;
  width: 30px;
  height: 3px;
  margin-right: 3px;
  margin-left: 3px;
  text-indent: -999px;
  cursor: pointer;
  background-color: #fff;
  background-clip: padding-box;
  border-top: 10px solid transparent;
  border-bottom: 10px solid transparent;
  opacity: .5;
  transition: opacity 0.6s ease;
}
bootstrap4.css
.border-top {
  border-top: 1px solid #dee2e6 !important;
}
bootstrap4.css
.border-top-0 {
  border-top: 0 !important;
}
bootstrap4.css
.rounded-top {
  border-top-left-radius: 0.25rem !important;
  border-top-right-radius: 0.25rem !important;
}
bootstrap4.css
.rounded-right {
  border-top-right-radius: 0.25rem !important;
  border-bottom-right-radius: 0.25rem !important;
}
bootstrap4.css
.rounded-left {
  border-top-left-radius: 0.25rem !important;
  border-bottom-left-radius: 0.25rem !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 -->