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15-card booster packs usually contain 3 uncommons. Some of the earliest expansions; including Arabian Nights, Antiquities, The Dark, Fallen Empires, and Homelands; were packaged in 8-card booster packs that usually contain 2 uncommons. The Alliances and Chronicles expansions were sold in 12-card booster packs that also usually contain 2 uncommons.
It is known that expansions through Alliances, excluding Ice Age, had some common and uncommon cards that were actually more prevalent than others. This was the result of some cards appearing more often than others on the printer's sheet of uncut cards. For example, Scarwood Bandits is a U1 uncommon, and Scarwood Hag is a U2 uncommon. This code represents the number of times the card appears on the uncommon printer's sheet of uncut cards, making U2 uncommons twice as abundant as U1 uncommons. Additionally, since 8-card booster packs contained no true rare cards, U1 uncommons are often referred to as the "rares" in these expansions.