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From MTG Wiki
The Ravnica block contains 10 theme decks each of which contain 2 colors representing their respective guilds. Ravnica block is the first block that doesn't have 12 theme decks. Each of the Ravnica block sets have theme decks that concentrate on the individual guilds in that set. In the two later sets there were 3 guilds so there were just 3 theme decks instead of the normal 4.