Field guide / Rule 14 — Numbers
Write a large number and two conventions collide. English groups the thousands with a comma and marks the decimal with a dot — 1,234.56. Much of the world does the exact reverse — 1.234,56. A document should pick one and hold it, and which one depends on its language.
The comma and the dot do opposite jobs on either side of an ocean. In English, a comma breaks the thousands (1,234,567) and a dot opens the decimal (3.14). In Brazil and much of Europe the roles swap — a dot groups (1.234.567) and a comma is the decimal (3,14). Neither is wrong; what jars a reader is a document that mixes them.
So we normalise numbers to the document’s language. English is the default, and an English document gets the English convention. A document that declares another language keeps that language’s numbers — and one whose language legitimately uses the other form is left untouched entirely.
The catch is that a single separator carries no proof of its job. 1.234 could be a thousand written the European way, or a decimal written the English way — the digits alone can’t say. Guess wrong and you turn 1,234 dollars into 1.234 dollars, a thousand-fold error. So the rule fixes only what is unambiguous — a number that shows both separators, or groups its thousands more than once — and leaves every lone separator exactly as you wrote it.
In an English document we set the numbers whose meaning is beyond doubt, and keep our hands off the rest — the ambiguous single separators, and anything that isn’t a quantity at all:
Both separators present — so the roles are clear, and we set the English form.
Thousands grouped more than once can only be grouping — regrouped with commas.
A currency amount with a comma decimal is set with a dot.
One separator, no context — ambiguous, so left exactly as written.
A version or a year is not a quantity — never regrouped.
In short
Pick one convention for your numbers and hold it — English uses a comma for thousands and a dot for the decimal. We normalise numbers to your document’s language wherever the meaning is unambiguous, and we leave lone separators, versions, dates and years alone, because there a guess could move a decimal point.
Want to see it on your own text? Convert a document and read the cleanup report — every number we reformatted is counted there.