Check out example codes for "css nth-child". It will help you in understanding the concepts better.

Code Example 1

/* Selects the second <li> element in a list */
li:nth-child(2) { 
  color: lime;
}

/* Selects every fourth element
   among any group of siblings */
:nth-child(4n) {
  color: lime;
}

Code Example 2

/* Select the first list item */
li:nth-child(1) { }

/* Select the 5th list item */
li:nth-child(5) { }

/* Select every other list item starting with first */
li:nth-child(odd) { }

/* Select every 3rd list item starting with first */
li:nth-child(3n) { }

/* Select every 3rd list item starting with 2nd */
li:nth-child(3n - 1) { }

/* Select every 3rd child item, as long as it has class "el" */
.el:nth-child(3) { }

Code Example 3

p:nth-child(2)
 	{     
 		background: red;
 	}

Code Example 4

/*Specify a background color for every <p> element that is the 
second child of its parent:*/
  
 p:nth-child(2)
 	{     
 		background: red;
 	}

Code Example 5

tr:nth-child(even) {background: #CCC}
tr:nth-child(odd) {background: #FFF}

Code Example 6

/* Select the first list item */
li:nth-child(1) { }
/* Select odd list items */
li:nth-child(odd) { }
/* Select even list items */
li:nth-child(even) { }
/* Select each list item every 3 (3,6,9...) */
li:nth-child(3n) { }
/* Select each list item every 3 starting from the fourth (4,7,10...) */
li:nth-child(3n+1) { }

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