Someone

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
<script src="jquery-1.2.1.min.js" type="text/javascript"></script>
</head>

<body>
  <div class="class1 class2"><h1>CONTENT!</h1></div>
</body>
</html>

FIREBUG:

>>> x = $('.class1')
[div.class1]
>>> y = $('.class2')
[div.class1]
>>> z = $('.class1 .class2')
[]