Most people who commonly use the imperial system of measurement are familiar with the different symbols used to designate inches and feet. However, we don’t usually stop to think about why these particular symbols are used. These symbols, however, have many uses and meanings.

**Foot and Inch Symbols**

A foot can be designated by the abbreviation ft. or by a prime mark, which looks very similar to a single apostrophe. An inch can be designated by the abbreviation in. or two prime marks, called a double prime, which looks like quotation marks. For example, if a person is 6 feet 3 inches tall, you can write it as 6 ft. 3 in., or 6′ 3″.

**The History of the Prime Mark**

Prime marks are, at their most basic, simply a way to count things. The marks were originally used in mathematics. The marks can mean a few different things, but they are most commonly used to describe similar things.

For example, you might have an x variable and a variable related to x. The variable related to x would be marked as x’. The next variable would be x” and so on.

The word ‘prime’ does not refer to a prime number. Instead, it’s an older way of saying ‘one’ or ‘first.’ A variable with one prime mark would be called x prime. A variable with two prime marks would be called x second.

Today, most people simply refer to one prime mark as a prime and two prime marks as a double prime.

In the case of feet and inches, the prime marks are ordered by how you would describe something’s height or length. Feet are said first, so feet are symbolized with the single prime mark.

Inches are described second, so they are symbolized with the ‘second’ or double prime mark. The smaller marks make it quicker to write out a measurement, and that measurement takes up less space and is often easier to read.

Normally, only one or two prime marks are used for measurements, but you might see a triple or treble prime mark once in a while. This mark is most commonly used by watchmakers to indicate a ligne. The ligne is also called a pouce or French inch, and it measures 0.089 inches.

**How A Prime Mark is Written**

Although apostrophes or quotation marks are usually used to write prime marks in modern times, an apostrophe and a prime mark are not actually the same. Prime marks should have a straight left edge and should slant slightly to the right.

Many people, particularly those who use formal typesetting, feel that these two marks should never be used interchangeably. However, most modern keyboards do not have a key for a prime mark, but only keys for apostrophes and quotations.

**Other Prime Mark Uses**

Prime marks are used for many applications aside from measurements. They can be used for mathematics in a variety of ways. Aside from marking variables, they can be used for angles.

A single prime mark denotes arcminutes, which are 1/60 of a degree, while a double prime indicates arcseconds or 1/60 of an arcminute.

Similar to the notations for arcminutes and arcseconds, single and double primes can be used to symbolize seconds or minutes of time. Minutes are indicated by a single prime mark while seconds are symbolized by a double prime.

In math, prime marks can also symbolize the result of a transformation, the transpose of a matrix, or the set complement. A single prime also designates the different functional groups that are connected to an atom in a molecule. Primes can also be used in biology.

A prime mark designates the position of carbon on a ring of ribose or deoxyribose, which can help scientists track the direction of movement as an enzyme moves along a string of DNA.

Prime marks are also used in music notation, particularly in the Helmholtz pitch notation system. This system uses a single prime to distinguish between notes in octaves from middle C up. A prime can also be used over a fermata mark to denote either a long rest or a long note.

Finally, prime marks can be used in linguistic settings. The symbol is often used to denote palatalization in some words, particularly in Slavic languages. Prime and double prime symbols are also sometimes used to denote the X bar symbol used in X bar theory.

**The History of the Foot and Inch**

Feet and inch measurements are derived from the average length of a person’s foot and thumb. People in ancient times could easily use their own feet or hands to measure things, but the system was not uniform.

Over time, the foot and inch marks became standardized, and the definition of the length of each was finally agreed upon in 1959.