FreeCell is a solitaire card game played using the standard 52-card deck. It is fundamentally different from most solitaire games in that very few deals are unsolvable, and all cards are dealt face-up from the very beginning of the game. Although software implementations vary, most versions label the hands with a number (derived from the seed value used by the random number generator to shuffle the cards). Microsoft has included a FreeCell computer game with every release of the Windows operating system since 1995, greatly contributing to the game's popularity. It is so definitive for many FreeCell players that many other software implementations strive for compatibility with its random number generator in order to replicate its numbered hands.