Field guide / Rule 04 — Spacing
The two-space gap after a full stop is one of the most persistent typing habits there is — and one of the clearest signs a document came off a typewriter, not a typesetter. In set text the answer is simple: one space, every time.
A typewriter gave every character the same width — an i took as much room as an m — so sentences ran together and a single space after a period barely registered. Typists were taught to hit the bar twice to open the gap and mark the end of a thought. It was a sensible fix for a real problem: monospaced type has no other way to breathe.
Proportional type — the type in every font you use today — never had that problem. The letters are spaced to sit correctly on their own, and the space after a period is already sized to end a sentence. Add a second one and you punch small white holes down the page, the “rivers” that pull a reader’s eye off the line. Every major style guide now sets a single space; the double is simply a habit outliving its machine.
You do not have to retrain your thumbs. Our first cleanup pass — the always-on one, before any typography is considered — collapses every run of spaces down to a single space. Two spaces after a period, three after a colon, a stray double mid-sentence: all become one. And if a sentence ran into the next with no space at all, we add the one it was missing.
The double space after the period collapses to one.
A missing space between sentences is added — lowercase then a capital.
A dot inside a name or URL is not a sentence — left untouched.
An initialism keeps its dots — capitals around the period are a signal to leave it.
In short
One space after a period — the double belongs to the typewriter. We collapse every extra space to a single one on the way out, add a space where a sentence lost it, and step carefully around URLs, filenames and initialisms so nothing that isn’t a sentence gets touched.
Want to see it on your own text? Convert a document and read the cleanup report — every space we collapsed is counted there.