The use of small tick marks to represent measurements often leads to confusion regarding which symbol stands for feet and which stands for inches. This shorthand notation is common in the Imperial and U.S. customary systems of linear measurement, particularly in construction and engineering. The marks are technically known as the prime ($\text{′}$) and double prime ($\text{″}$) symbols, though they are frequently substituted with the single apostrophe ($\text{‘}$) and double quotation mark ($\text{“}$) due to limited keyboard options. Understanding the distinction clarifies how to correctly interpret and write physical dimensions.
The Symbol for Feet and Inches
The single prime symbol ($\text{′}$) stands for the unit of feet, while the double prime symbol ($\text{″}$) is the standardized shorthand for inches. This convention provides a concise way to express length measurements without needing to write out the full abbreviations $\text{“ft”}$ and $\text{“in”}$. The system originates from a mathematical concept where the symbols indicate successive divisions of a unit. Feet are the first major division of a yard, and inches are the second, or the division of a foot, which is why the single and double marks are applied respectively.
Proper Notation for Measurement
When writing a measurement that includes both feet and inches, the standard practice is to list the larger unit first, followed by the smaller unit. For instance, a height of five feet, ten inches is correctly represented as $\text{5′ 10″}$. In many technical contexts, the number for feet and the number for inches are placed immediately next to each other with no space, such as $\text{5′10″}$. This compact format ensures clarity, especially in blueprints or technical specifications where space is limited. The symbol for feet is placed directly after the numerical value for feet, and the symbol for inches is placed immediately after the numerical value for inches.
Prime Symbols in Other Contexts
The single and double prime symbols are not exclusive to linear measurements, as their meaning shifts entirely based on the context in which they appear. These marks are regularly used in the measurement of angles, such as in geography, astronomy, and navigation. Here, the single prime ($\text{′}$) signifies a minute of arc, also known as an arcminute, which is one-sixtieth of a degree. Following this logic, the double prime ($\text{″}$) represents a second of arc, or arcsecond, which is one-sixtieth of an arcminute.
The single mark is also used in standard English for possessives or to indicate a contraction, which often contributes to typing confusion when the prime symbol is unavailable. The symbols can even denote minutes and seconds of time, a convention sometimes seen in the naming of musical compositions.