Glossary
Inner sleeve
Also called: perfect fit, perfect fit sleeves, inner sleeves
An inner sleeve is a snug, thin sleeve sized close to the card itself, worn underneath a normal deck sleeve to seal the card in from both ends.
"Perfect Fit" is Dragon Shield's name for the format and has become the common term. Their Perfect Fit inner sleeves are made for 63 × 88 mm cards, are polypropylene, and the thick version is 100 µm.
The point is closure. A standard sleeve is open at the top, so dust and moisture can still reach the card. Put the card in an inner sleeve first, insert it upside down into the outer sleeve, and both openings are covered.
The trade-off is fit. Two layers of plastic make the card measurably thicker, which affects how a deck shuffles and whether it still fits a deck box rated for single-sleeved cards.
Inner sleeves come in sealable and non-sealable versions; sealable ones close all four edges at the cost of being fiddlier to load.