Two Meanings for "If Not"

The phrase "if not" has two distinct senses, and it is not always clear which one is intended.

1) "and maybe"; "or perhaps". Extended warranties, obscenely marked-up cables and the Geek Squad constitute most, if not all, of Best Buy's profit.

2) "though not (quite)". The iPad is clearly a game-changer, if not the death of the PC, as some have asserted.