If you've looked at our Arabian Nights card list (here), you may be wondering what the rarities codes are.
Arabian Nights did not have three print sheets of cards like current sets do (four, if the set includes basic lands). Instead there was only a common and uncommon sheet, and the cards were on those sheets in different frequencies. Each sheet contained 121 cards.
The rarity code for each card consists of a letter that tells you which sheet it was on, and a number that tells you how many times it appeared. The three cards shown below are all commons.
So for every five Unstable Mutation cards printed, four War Elephant cards were printed.
The most common card? Desert, which has a rarity of C11. The only card to appear one time on a sheet? The Arabian Mountain, which has a rarity of C1, making it just as hard to find as cards with a rarity of U3. (The common-uncommon ratio was 3:1 in the booster packs.)