Check out example codes for "border bottom class in bootstrap 4". It will help you in understanding the concepts better.

Code Example 1

<span class="border"></span>
<span class="border-top"></span>
<span class="border-right"></span>
<span class="border-bottom"></span>
<span class="border-left"></span>

Code Example 2

<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>

Code Example 3

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">

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