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A halving series of sheet sizes, from A0 at one square metre downward.
Also known as: A4 paper size, Metric paper sizes
The A series of paper sizes is built on two rules. Every sheet has sides in the ratio of one to the square root of two, which is the only ratio preserved when a sheet is halved across its longer side; and A0 has an area of exactly one square metre. Everything else follows: A1 is half of A0, A4 is a sixteenth, and enlarging A4 to A3 is a scale factor of about 1.414 rather than an awkward number. North American sizes follow neither rule, which is why documents do not travel between the two systems cleanly.
Uses a side ratio of one to the square root of two, which is the only ratio that survives halving a sheet across its longer side — so every size in the series has the same proportions as every other.
Contexts: News, Technical drawing
Fixes A0 at exactly one square metre, which is what turns the ratio into actual dimensions and makes paper weight in grams per square metre directly comparable across sizes.
Contexts: News, Technical drawing
Has no counterpart in North American sizes, which follow neither rule, so a document laid out for one system does not scale cleanly into the other — the recurring reason a page prints with unexpected margins.
International and North America · Contexts: News, Technical drawing
A nested set of rectangles each half the area of the one before, arranged so that the longer side of each becomes the shorter side of the next, labelled with a letter and a number.
Both belong to the vocabulary of preparing a job for print.