If you want to match only elements with both classes (an intersection, like a logical AND), just write the selectors together without spaces in between:

$('.a.b')

The order is not relevant, so you can also swap the classes:

$('.b.a')

So to match a div element that has an ID of a with classes b and c, you would write:

$('div#a.b.c')

(In practice, you most likely don't need to get that specific, and an ID or class selector by itself is usually enough: $('#a').)