When you first receive a W-2 you will notice there are four exact W-2 forms attached together .Those are each referred to as copy A-B-C-D , or sometimes copy 1-2-3-4. You need to attach either copy A or B to the front of your 1040a form when you mail it in.You can usually find the letter/number at the top or the bottom of your W-2.Each copy is for different filing such as one for Federal,2 for State and local, and one for your record.