Field guide / Rule 03 — Prime marks
Feet and inches, minutes and seconds, latitude and longitude are not measured in quotation marks — they take primes. It is the one case where every smart-quote feature reliably reaches for the wrong mark, and the one mark we leave for your own hand.
The prime (′) and double prime (″) are marks of quantity, not of speech. A single prime means feet, or minutes of arc, or minutes of time; a double prime means inches, or seconds. So a height is 5′11″, a coordinate is 40°26′46″ N, and a run time is 3′42″. They lean to the right at a slant — related to the italic, not to the curl of a quotation mark.
The reason they matter is that a straight quote (") is often used as a lazy stand-in — 5'11" — and once smart quotes touch it, that stand-in turns into a curly quotation mark, which is simply the wrong character in the wrong place.
A smart-quote feature has one job: decide whether a straight mark opens or closes a quotation, and curl it accordingly. Feed it a measurement and it does exactly that — it sees 5'11", curls the ' into an apostrophe and the " into a closing quote, and hands you 5’11”. Every glyph is now a speech mark, and not one of them is a prime.
The trouble is that the two are genuinely indistinguishable from the text alone. 5″ of pipe and a passage that ends on 5" of quoted dialogue are the same keystrokes in the same place; only the author knows a length was meant. That is why this is the one mark the previous rule set aside — the case a machine cannot safely call.
This is the one place we deliberately step back. Our smart-quote pass will curl the straight marks in a measurement just like any other quote, because it cannot know a length was intended — so if your document measures things, set the primes yourself with the real characters, ′ and ″. What we do promise is that a prime you have already set is safe: we never touch ′ or ″, and we never curl them into quotes.
One thing we will not do is talk you into a substitute. It is common advice to fall back on straight quotes when a font is missing the prime glyph — we take the other view: a measurement deserves the real mark, so choose a typeface that carries a true ′ and ″ and never accept one that doesn’t. A tick standing in for a prime is just a second-hand substitute, and your reader can tell.
Straight marks curl to quotes, not primes — set 5′11″ by hand.
Real primes are left untouched — we never curl a mark you set right.
Minutes and seconds, already set as primes — kept exactly as written.
In short
Measurements take primes, ′ and ″ — never quotes. It is the one mark no software can guess from the text, so we hand it to you: set your feet, inches and coordinates with the real characters, and trust us to leave every prime you set alone.
Curious how we handle the marks we can call? See straight and curly quotes, where this exception first comes up.