Border utilities provide control over element borders:

<!-- Add borders -->
<div class="border">All borders</div>
<div class="border-top border-start">Top and left borders</div>

<!-- Remove borders -->
<div class="border border-top-0">All except top border</div>

<!-- Border colors -->
<div class="border border-primary">Primary border color</div>

<!-- Border radius -->
<div class="rounded">Rounded corners</div>
<div class="rounded-circle">Circular border</div>
<div class="rounded-pill">Pill-shaped border</div>

Border sides: border-top, border-end, border-bottom, border-start.
Border radius: rounded-top, rounded-end, rounded-bottom, rounded-start, rounded-circle, rounded-pill.