CSS Pseudo-class :nth-child(an+b)
Selects every nth child-element by using the formula an+b. a and b are integers. If you want to select every fourth child-element of
body starting with the first child, you write
body > *:nth-child(4n+1). This will select the first, the fifth, the ninth (and so on) child-element of
body. If you want to select the fourth, the eighth, the twelfth and so on, you just write
body > *:nth-child(4n). You also can use negative numbers for b.
body *:nth-child(4n-1) will select the third, seventh, eleventh, fifteenth... child-element of
Instead of an+b you also can use two keywords: even and odd which are equal to 2n and 2n+1.