The ″ Double Prime Symbol: Meaning, Code, and Usage
The double prime symbol (″) is a typographical mark primarily used to represent inches in the imperial measurement system and angular seconds in mathematics. While it frequently gets confused with a standard set of double quotation marks ("), the double prime is slightly slanted and serves a distinct scientific purpose rather than a grammatical one.
Prime symbols originated in historical mathematics to indicate the divisions of units. A single prime represented the first division, such as feet or minutes, while the double prime represented the second division, such as inches or seconds. In the Unicode standard, the Double Prime is officially assigned the code point U+2033 and resides within the "General Punctuation" block.
You will spot the double prime most often in real-world measurements. If you are looking at the specs for a new smartphone or a television, a "65″ display" relies on this exact mark. In geometry and navigation, it denotes arcseconds when measuring angles, formatted like 45° 15′ 30″ (45 degrees, 15 minutes, and 30 seconds). In higher mathematics like calculus, a double prime is used to signify the second derivative of a function, famously written as f″(x).
Most physical keyboards do not feature a dedicated key for the double prime, which explains why everyday internet users simply type standard straight quotes instead. However, if you need precise typography for a project, you can insert it manually. On Windows, hold the Alt key and type 8243 on the numeric keypad. On a Mac, press Control + Command + Space to open the Character Viewer and search for "double prime". Web developers can seamlessly render it in HTML by typing the entity ″ or ″.
The double prime is part of a larger family of mathematical marks, sitting directly between the single prime (′, U+2032) and the elusive triple prime (‴, U+2034). Its most common mix-ups are with the neutral double quote (", U+0022) and curly, or "smart", quotation marks (“ ”). Typography enthusiasts are quick to point out that using a standard straight quote for inches is a major design faux pas. Conversely, in programming, using a true double prime instead of an ASCII quote will almost certainly break your code by throwing a syntax error. Keep the double prime reserved for your visual layouts, architectural blueprints, and scientific publishing.