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

Code Example 1

.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 2

#roundCornerID {
    border-radius: 30%;
    width: 200px;
    height: 150px;   
}

Code Example 3

#rcorners {
  border-radius: 25px;
}

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