Sleeve
A sleeve is a clear plastic protector slid over a card to prevent wear and tear, hide back markings, and standardize feel across a deck. Sleeve sizes are matched to card sizes — poker sleeves fit poker cards, mini sleeves fit mini cards, and so on.
Why it matters
Sleeves matter at design time because a sleeved deck is fatter than a bare deck — about 0.6 mm extra per card. A 54-card deck in a tuck box becomes a 54-card sleeved deck that no longer fits the same box.
Example
Standard poker sleeves are 66 × 91 mm — a hair larger than the 63.5 × 88.9 mm card inside. If your game expects sleeves, account for that extra width when sizing the box.