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').)