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CSS3 provides powerful pseudo-classes that allow the CSS designer to select multiple elements according to their positions in a document tree. The :nth-child(an+b) CSS pseudo-class matches a number of child elements whose numeric position in the series of children matches the pattern an+b. The argument, n, can be a keyword, a number, or a number expression of the form an+b. Think of "n" as starting at zero and then a set of all positive integers. Then complete the expression. So the 2n is "2xn", and the whole expression together is "(2xn)+3".