The :nth-child(an+b) pseudo-class notation represents an element that has an+b-1 siblings before it in the document tree, for a given positive integer or zero value of n. In other words, this matches the bth child of an element after all the children have been split into groups of a elements each. For example, this allows the selectors to address every other row in a table, and could be used, for example, to alternate the color of paragraph text in a cycle of four. The a and b values must be zero, negative integers or positive integers. The index of the first child of an element is 1.

for a given positive integer or zero value of n