How do I show elements having two classes?

How do I show elements having two classes?

Right now I have something of the form in my CSS:

.a { display: none }

and in my HTML

<tr class="a b">...

How do I show all items belong to classes "a" and "b"? Right now I'm trying:

$('a,b').show();

Is this the right approach, or should I be doing something else?