The inch mark is a double prime
The correct inches symbol is the double prime ″ (U+2033), a pair of slanted strokes. It pairs with the single prime ′ for feet, so a person who is 5′ 11″ is five feet eleven inches tall. One inch is exactly 2.54 cm, and 12 inches make a foot.
The same mark has other jobs: it means seconds of arc in angles and map coordinates (40° 26′ 46″), and in math it marks a second derivative, as in f″(x). Screen sizes like 27″ and 65″ are measured across the diagonal.
″ vs the quotation mark
On a keyboard, most people type the straight quote " for inches. That is fine in plain text and code, but word processors and phones often "smart quote" it into a curly ” or “, which is wrong for measurements: 27” looks like the end of a quotation. The double prime ″ avoids that problem and looks right in print.
Other look-alikes are different characters, too. ʺ (modifier letter double prime) is used to transliterate Cyrillic, 〃 is the East Asian ditto mark, and ˝ is a double acute accent. Search and spell check will not treat any of them as inches. For real quotation marks, see quotation mark symbols.
How to type the inches symbol
Windows has no classic Alt code for ″. In Word, type 2033 and press Alt + X. If Word turns your " into a curly quote, press Ctrl + Z right away to undo just that change and keep a straight ".
On a Mac, open the Character Viewer (Control + Command + Space) and search for "double prime". On iPhone, turning off Smart Punctuation in Settings > General > Keyboard keeps " straight; for the true double prime, copy ″ from this page.
In HTML, use ″ with a capital P; ′ is the single prime for feet. In LaTeX math mode, f'' gives a double prime.