border bottom class in bootstrap 4

HTML
<span class="border"></span>
<span class="border-top"></span>
<span class="border-right"></span>
<span class="border-bottom"></span>
<span class="border-left"></span>Border-radius
Add classes to an element to easily round its corners.

<img src="..." alt="..." class="rounded">
<img src="..." alt="..." class="rounded-top">
<img src="..." alt="..." class="rounded-right">
<img src="..." alt="..." class="rounded-bottom">
<img src="..." alt="..." class="rounded-left">
<img src="..." alt="..." class="rounded-circle">
<img src="..." alt="..." class="rounded-0"><span class="border">Text</span>
<span class="border-top">Text</span>
<span class="border-right">Text</span>
<span class="border-bottom">Text</span>
<span class="border-left">Text</span>

Source

Also in HTML: