What's the difference between A2, A6 and A7 envelopes?

Size. A2 (4 3/8" x 5 3/4") fits 4.25" x 5.5" cards, A6 (4 3/4" x 6 1/2") fits 4.5" x 6.25" cards, A7 (5 1/4" x 7 1/4") fits 5" x 7" invitations, and A9 (5 3/4" x 8 3/4") fits 5.5" x 8.5" cards.