Question : jquery selector basic question

Hi, I know that jquery selector accepts CSS syntax etc. I am puzzled however by this
example for going through each  column:

Why in  $('th', $table) we need a comma and then $table object  ?

Would not be enough just to put $('table th') ?
 or just $('th') ?

$(document).ready(function() {
$('table.sortable').each(function() {
var $table = $(this);
$('th', $table).each(function(column) {
var $header = $(this);
if ($header.is('.sort-alpha')) {
$header.addClass('clickable').hover(function() {
$header.addClass('hover');
}, function() { ...
$header.removeClass('hover');

Answer : jquery selector basic question

FYI: when you do this:
 $('th')

internally, jquery actually "executes":
 $('th', document);

that's why you would get all the th in the document.

Reference:
http://docs.jquery.com/%24