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On memory cards, blocks represent space usable for game saves. For example, a save from Animal Crossing uses 57 blocks on a Gamecube memory card. If the save were on a 59-block memory card, this would leave 2 blocks left for a save that takes up only 1 or 2 blocks.
In most games, blocks are platforms or obstacles that the player can use. For this definition, also see crate.