jQuery Selectors Advanced
The jQuery nth-child is used to select all elements that are ntg-child of of their parent. The nth-child(n) is “1-indexed”, meaning the “n” is counting starts at 1.
Because jQuery's implementation of :nth-child(n) is strictly derived from the CSS specification, the value of n is "1-indexed", meaning that the counting starts at 1. For all other selector expressions, however, jQuery follows JavaScript's "0-indexed" counting.