Air pillow
Also called: pool pillow, winter cover pillow, ice compensator.
An air pillow is an inflatable cushion tied off at the centre of an above-ground pool under the winter cover. Its documented job is holding the cover up out of the gap between the top rail and the lowered water line, so the cover doesn't sag in and fill.
Why it matters
It also crowns the middle so runoff slides to the edges. Its makers sell it as ice protection too: Blue Wave heads a bullet "Ice Equalization" and Swimline calls its pillows "ice compensators". Nobody independent has shown that a pillow measurably protects a pool wall, and pillows routinely go flat mid-winter, so a pillow that is flat in January cushions nothing. What protects an above-ground pool from ice is clearing and lowering the water in the skimmer line. Treat a pillow as a consumable. Fill it 50–70%, never full: POOLHACKER's instructions give that range, Leslie's says about halfway, and an over-inflated pillow is the one that splits.
In numbers
A standard round pool takes one 4×4; a large round takes an 8×8, or two 4×4s spaced across the span. Ovals usually need more than one, and In The Swim's rule is enough pillow to cover about 10% of the water surface.