Check out example codes for "css circle border". It will help you in understanding the concepts better.

Code Example 1

.circle {
    background-color:#fff;
    border:1px solid red;    
    height:100px;
    border-radius:50%;
    -moz-border-radius:50%;
    -webkit-border-radius:50%;
    width:100px;
}
<div class="circle"></div>

Code Example 2

.div { /*Top Left (TL) Top Right (TR) Bottom Right (BR) Bottom Left (BL)*/
	border-radius: 15px 50px 30px 5px; /*TL TR BR BL*/
  	border-radius: 15px 50px 30px; /*TL TRBL BR*/
  	border-radius: 15px 50px; /*TLBR TRBL*/
  	border-radius: 15px; /*All corners*/
}

Code Example 3

#rcorners {
  border-radius: 25px;
}

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