The number-formats lesson established the constitution: values are truth, display is costume, and the costume can’t touch the numbers. This lesson is the tailoring course. Behind Ctrl+1 → Custom sits a small pattern language — one line of symbols that controls exactly how a value dresses — and knowing a dozen of its words replaces a hundred ugly workarounds.

The grammar: four sections, one line

A custom format is up to four patterns separated by semicolons:

positive ; negative ; zero ; text

Give fewer and Excel fills sensibly (one pattern covers everything). The accountant’s classic uses all four ideas at once:

#,##0;[Red](#,##0);"—";@

Positives with thousands separators; negatives red and bracketed; zeros as a quiet dash; text passed through (@ means “the text itself”). One line, and a column reads like a published report — while every underlying value stays exactly what the formulas need.

The vocabulary is smaller than it looks: 0 = always show a digit (pad if needed), # = show if present, , = thousands separator, . = decimal point, "text" = literal text, [Red]/[Blue] = colour, @ = the text value. That’s most of the language.

The recipes worth stealing

Units without sin. 0.0" kg" displays 2.3 kg while the cell holds 2.3 — still summable, still chartable. Typing 2.3 kg as text is the beginner version; it looks the same and breaks every calculation. The format is how you get both.

Thousands and millions, honestly. A trailing comma divides the display by a thousand: #,##0, shows 48,213,062 as 48,213; #,##0,,"M" shows it as 48M. The value never moves — the honest big-figures trick from the first formats lesson, now yours to write.

Padded IDs. 00000 displays 42 as 00042 — the answer to lost leading zeros when the values are genuinely numbers. (IDs that should never be arithmetic at all belong as text from import; this recipe is for counters you compute.)

Dates with a day name. ddd dd mmm → Wed 16 Jul; mmmm yyyy → July 2026. Every date column in a print-ready report deserves a deliberate date costume rather than the default.

Hiding, carefully. ;;; (three semicolons, nothing else) displays nothing at all — the cell looks empty while holding its value. Occasionally right (a helper flag that only conditional formatting reads); mostly a trap for the next reader — prefer a hidden column, which at least announces itself.

the value -48213.5 held once, never edited the costumes (48,214)#,##0;[Red](#,##0) -48.2 t0.0" t" — units, still a number -£48k£#,##0,"k" — display ÷ 1,000 -48,213.50#,##0.00 — the plain suit four costumes on rotation; the wardrobe line is one field in Ctrl+1 → Custom — and SUM sees the same number under every one of them
One value, a wardrobe of one-line patterns. The tailoring never touches the number — which is the entire constitution of formatting.

The boundaries

Three, honestly held. Conditions beyond colour belong to conditional formatting — the language allows [>1000]-style tests, but rules written in Manage Rules are visible and maintainable; format-string logic is a message in a bottle. Text-heavy displays belong to formulas — if the costume needs TEXT()-level gymnastics, build the display string in its own column and keep the value pure beside it. And the reader outranks the tailor: a format that makes a colleague ask “is that thousands?” failed, however clever — chart-title rules apply to number costumes too. Label the units somewhere the eye lands.

Four sections, a dozen symbols, five recipes. The costume department turns out to have a workshop — and every stitch of it obeys the first law you learned: the value underneath never moves.