jQuery :nth-child() Selector

Because jQuery’s implementation of :nth- selectors is strictly derived from the CSS specification, the value of n is "1-indexed", meaning that the counting starts at 1. For other selector expressions such as :eq() or :even jQuery follows JavaScript’s "0-indexed" counting. Given a single <ul> containing two <li>s, $( "li:nth-child(1)" ) selects the first <li> while $( "li:eq(1)" ) selects the second.

The :nth-child(n) pseudo-class is easily confused with :eq(n), even though the two can result in dramatically different matched elements. With :nth-child(n), all children are counted, regardless of what they are, and the specified element is selected only if it matches the selector attached to the pseudo-class. With :eq(n) only the selector attached to the pseudo-class is counted, not limited to children of any other element, and the (n+1)th one (n is 0-based) is selected.

Further discussion of this unusual usage can be found in the W3C CSS specification.