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 = $(this);
if ($'.sort-alpha')) {
$header.addClass('clickable').hover(function() {
}, function() { ...

Answer : jquery selector basic question

FYI: when you do this:

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

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

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