nth-child

/* 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;
}

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

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

:nth-child(3) { //the number is the child number you are targeting
	//styles here 
}

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

/* 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) { }

li:nth-child(even) { /* Selects only even elements */
    color: green;   
}

  
    p:nth-child(2)
 {

     
 background: red;

 }
  
 

li:nth-child(odd) { /* Selects only odd elements */
    color: green;   
}