"A3", "B1", and "Legal" are typical names of sizes for what object?

The story behind the answer

Paper. “A3” and “B1” come from ISO 216, the international paper standard: the A-series (A0, A1, … A4, A5) has a constant √2:1 aspect ratio, with A0 defined as 1 m²; each step (A0→A1→A2…) halves the area. Thus A3 is twice the size of A4. The B-series (B0, B1, …) provides intermediate sizes—useful for posters and envelopes—sitting between adjacent A sizes.

“Legal” is not an ISO size; it’s a North American standard measuring 8.5 × 14 inches, related to “Letter” (8.5 × 11) and “Tabloid” (11 × 17). Memory tip: “A and B are ‘Around the world’ (ISO);