Non-breaking space

U+00A0 looks exactly like an ordinary space and behaves like one in every way except the one that matters: a line breaker is not allowed to break there. In HTML it is written  , and it is the oldest piece of typographic glue on the web. Everything else on this page is about where lines are allowed to fall; this is the character that says not here.

The recurring cases are all pairs that mean nothing apart. A number and its unit (10 km, 4 GB, £12 million), a title and a name (Dr. Chen), a label and its reference (Figure 3, section 4.2, chapter 9), initials in a name, and a short word you do not want stranded at the end of a line before a link. The diagnosis is easy: if seeing the two halves on different lines would make you read the line twice, glue them. The soft hyphen is the mirror image of this character, one permits a break where there was none and this one forbids a break where there was one, and a well-set narrow column usually needs both.

It is not a layout tool, though it has been used as one for thirty years. Runs of non-breaking spaces do not collapse the way ordinary whitespace does in HTML, which is why they became the poor typist’s indent, and why they fall apart at every width other than the one they were tuned on. Padding is the answer there. The real caution with legitimate use is overflow: nothing is permitted to break a glued phrase, so in a column narrower than the phrase it will simply stick out. Glue the pairs that need it, not every pair you can find.

Two neighbours are worth knowing. The narrow no-break space (U+202F) is the same promise at a tighter width, which some house styles prefer between a number and its unit. The word joiner (U+2060) forbids a break while taking up no width at all, which is what you want between a word and a following bracket or slash. And because all three are invisible in source, applying them consistently is a build step or a lint rule rather than a habit, in the same way that smart quotes end up being handled by a tool rather than by remembering.