Abstract: A breakdown air mattress assembly (A) is illustrated which includes a mattress (B) enclosed within a foam perimeter deck (C) which are both enclosed within a mattress cover (10). Deck (C) comprises individual deck sections (40a, 40b) which comprise a first side wall (32). Second deck sections (42a, 42b) which comprise a second side wall (34). Front deck sections (44a, 44b) comprising a front wall (36); and back deck sections (46a, 46b) comprising a back wall (38). Dissimilar joint means (D, E, F) join together the respective deck sections so that assembly may be had only in a prescribed and correct manner. A web (90) joins front and back sections (36, 38) together to cover a longitudinal joint (22) formed between dual mattresses (12, 14) in the case of a queen size or king size bed arrangement.