This is a playground to see how the selector works with different strings. Notice that this is different from the :even and :odd which have no regard for parent and just filter the list of elements to every other one. The :nth-child, however, counts the index of the child to its particular parent. In any case, it's easier to see than explain so...

Edit code and click me	Your Result (Some examples do not work in Google Chrome):
<!DOCTYPE html> <html> <head> <style> button { display:block; font-size:12px; width:100px; } div { float:left; margin:10px; font-size:10px; border:1px solid black; } span { color:blue; font-size:18px; } #inner { color:red; } td { width:50px; text-align:center; } </style> <script src="http://code.jquery.com/jquery-1.5.js" type="text/javascript"></script> <script type="text/javascript" language="javascript"> $(document).ready(function(){ $("button").click(function () { var str = $(this).text(); $("tr").css("background", "white"); $("tr" + str).css("background", "#ff0000"); $("#inner").text(str); }); }); </script> </head> <body> <div> <button>:nth-child(even)</button> <button>:nth-child(odd)</button> <button>:nth-child(3n)</button> <button>:nth-child(2)</button> </div> <div> <button>:nth-child(3n+1)</button> <button>:nth-child(3n+2)</button> <button>:even</button> <button>:odd</button> </div> <div><table> <tr><td>John</td></tr> <tr><td>Karl</td></tr> <tr><td>Brandon</td></tr> <tr><td>Benjamin</td></tr> </table></div> <div><table> <tr><td>Sam</td></tr> </table></div> <div><table> <tr><td>Glen</td></tr> <tr><td>Tane</td></tr> <tr><td>Ralph</td></tr> <tr><td>David</td></tr> <tr><td>Mike</td></tr> <tr><td>Dan</td></tr> </table></div> <span> tr<span id="inner"></span> </span> </body> </html>
Edit the code above and click to see the result	www.xhtml.co.il - Try it yourself