THE UTF-8 ENCODING

how a Unicode code point falls into one, two, three or four bytes — and which bytes never appear

Built as a memo rather than a table: the thing worth hanging it up for reads across the room, with the full reference in smaller type below it.

Three sizes in one archive — from a sheet on the fridge to a poster on the wall.

Formats: A4 · A3 · A2
What you get: Print-ready PDFs, one per size, with every letterform converted to outlines — nothing to install, nothing to shift.
Subject: code · reference · study

The first byte of a character says how many bytes it has; every byte after it opens with 10. The map gives the role of each of the 256 byte values, from ASCII to those that never occur in valid UTF-8.

SIX BYTES THAT ARE GONE

The first edition of UTF-8 reached six bytes and code points up to U+7FFFFFFF. RFC 3629 cut it back to four bytes and to U+10FFFF — exactly as far as UTF-16 reaches. Since then 13 byte values (C0–C1 and F5–FF) never appear in valid text at all.

It was cut with room to spare: by its own pattern the four-byte form would hold 2,031,616 code points, and 1,048,576 of them are valid — a little over half. In all, 1,112,064 code points can be encoded.

On the sheet

The first byte of a character says how many bytes it has; every byte after it opens with 10. The map gives the role of each of the 256 byte values, from ASCII to those that never occur in valid UTF-8.
Colour carries meaning, but meaning never rests on colour alone — the sheet reads in black and white.
Nothing about how it was made: the sheet carries only what the reader needs.

Printing

Three sizes in the archive: A4 — 210×297 mm, A3 — 297×420 mm, A2 — 420×594 mm.
All text is converted to outlines. No fonts to install, and nothing shifts on another machine.
White background and sparing fills — it prints on a home printer without draining the cartridge.
Margins of at least 10 mm: a home printer cannot print to the edge.
No stroke thinner than 0.3 mm, so nothing disappears in print.

Why the numbers can be trusted. Every countable number here is computed from data rather than typed in, and the sheet must pass 8 independent checks before it is allowed to build — how many items there are, the order they come in, and the claim in the highlighted box itself. If one fails, there is no file.

THE UTF-8 ENCODING

SIX BYTES THAT ARE GONE

On the sheet

Printing

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