The symbols used to denote inches are the double prime for shorthand, and the scientific "in." The double prime is often substituted with quotation marks, as both consist of a pair of small tick-marks similar to apostrophes. It is not to be confused with the unpaired prime, which indicates feet.
Because the inch is only used as a common unit of measurement in the United States, the scientific standard "in" is used much more frequently internationally than the double prime. Even within the United States, the double prime is often confused for the singular prime, occasionally making the traditional shorthand system confusing.