In measurements, the double quotation mark-like symbol (″) means inches, and the single mark (′) means feet. So a measurement written as 5′ 10″ means 5 feet, 10 inches. These same symbols also appear in angles and geographic coordinates, where they represent arcminutes and arcseconds. While people casually call them “quotation marks,” they’re technically different characters called prime and double prime symbols.
Feet, Inches, and How to Read Them
The most common place you’ll encounter these symbols is in everyday length measurements using the US customary system. The single mark after a number means feet, and the double mark means inches. A board that measures 6′ 4″ is 6 feet, 4 inches long. A box described as 4″ is 4 inches. There’s no space between the number and the symbol.
You’ll see this notation on building plans, woodworking projects, product dimensions, and height descriptions. When someone writes that they’re 5′ 9″, that’s 5 feet, 9 inches tall. The format is consistent: the single mark always follows the feet number, and the double mark always follows the inches number.
Angles and Geographic Coordinates
The same symbols pull double duty in angular measurement. When describing angles or positions on a map, the single prime (′) stands for arcminutes and the double prime (″) stands for arcseconds. These are subdivisions of a degree: one degree contains 60 arcminutes, and one arcminute contains 60 arcseconds. That makes arcseconds extremely small. There are 1,296,000 of them in a full circle.
An angle written as 3° 5′ 30″ reads as 3 degrees, 5 arcminutes, and 30 arcseconds. You’ll run into this format in GPS coordinates, astronomy, surveying, and navigation. A latitude like 40° 44′ 54″ N pinpoints a location to within about 30 meters on Earth’s surface.
Prime Symbols vs. Actual Quotation Marks
Here’s a detail that matters more than you might expect: the symbols used in measurements aren’t technically quotation marks at all. They’re called prime (′) and double prime (″) symbols, and they have their own dedicated spots in the Unicode character set. Unicode, the international standard for text characters, formally defines the prime symbol (′) as representing “minutes, feet” and the double prime (″) as representing “seconds, inches.”
True prime symbols are straight, slightly angled tick marks. Quotation marks, by contrast, come in two varieties: curly (also called “smart” quotes, like ” and “) and straight (” and ‘). The Chicago Manual of Style specifically warns against substituting double quotation marks or straight quotes for the double prime symbol, calling them different characters entirely.
In practice, most people type whatever their keyboard gives them. On a standard keyboard, pressing the apostrophe or quotation mark key produces a straight mark that looks close enough for casual use. Word processors often auto-convert these into curly quotes, which curve in a direction that looks wrong for measurements. If you’re writing something formal or technical, the correct approach is to insert the actual prime and double prime characters using your software’s special character menu.
Why These Symbols Exist
The prime notation has deep roots in a base-60 number system. Historically, mathematicians used the prime mark to show successive divisions by 60. A degree divides into 60 arcminutes (marked with one prime), and each arcminute divides into 60 arcseconds (marked with two primes). The 17th-century mathematician John Wallis even extended this system further, using triple and quadruple primes to represent “thirds” and “fourths” of arc, each one-sixtieth of the previous unit. Modern usage replaced those with decimal fractions, but the single and double prime survived.
The connection to feet and inches follows the same logic. Since a foot contains 12 inches, and these were the primary and secondary subdivisions of a yard in everyday use, the prime and double prime became convenient shorthand. An even more obscure example: in watchmaking, a triple prime (‴) represents a ligne, which is one-twelfth of a French inch, roughly 2.26 millimeters.
Quick Reference
- Single mark (′): feet (length) or arcminutes (angles)
- Double mark (″): inches (length) or arcseconds (angles)
- Degree symbol (°): degrees, used alongside the prime marks in angular notation
Context almost always makes the meaning clear. If you’re reading a product listing or a building plan, the marks mean feet and inches. If you’re looking at coordinates or an astronomy chart, they mean arcminutes and arcseconds. The symbols themselves are identical in both cases.