ABC-Box is a puzzle game invented by Inaba Naoki. The objective is to fill the center grid with the letters A, B and C to match the hints at the top and left. Any hint letter stands for one or more occurrences of that letter in the center grid. For example, a solution for the hint ABC in a 5x5-game could be AAABC, AABBC, AABCC, ABBBC, ABBCC or ABCCC. In the medium and hard modes a question mark can be any hint letter. The hint A?A could be ABA or ACA (but not AAA because that would simply be A).