Fraction

A fraction (from Latin: fractus, "broken") represents a part of a whole or, more generally, any number of equal parts. When spoken in everyday English, a fraction describes how many parts of a certain size there are, for example, one-half, eight-fifths, three-quarters. A common, vulgar, or simple fraction (examples: ⁠1/2⁠ and ⁠17/3⁠) consists of an integer numerator, displayed above a line (or before a slash like 1⁄2), and a non-zero integer denominator, displayed below (or after) that line. If these integers are positive, then the numerator represents a number of equal parts, and the denominator indicates how many of those parts make up a unit or a whole. For example, in the fraction ⁠3/4⁠, the numerator 3 indicates that the fraction represents 3 equal parts, and the denominator 4 indicates that 4 parts make up a whole. The picture to the right illustrates ⁠3/4⁠ of a cake.

Fractions can be used to represent ratios and division. Thus the fraction ⁠3/4⁠ can be used to represent the ratio 3:4 (the ratio of the part to the whole), and the division 3 ÷ 4 (three divided by four).

We can also write negative fractions, which represent the opposite of a positive fraction. For example, if ⁠1/2⁠ represents a half-dollar profit, then −⁠1/2⁠ represents a half-dollar loss. Because of the rules of division of signed numbers (which states in part that negative divided by positive is negative), −⁠1/2⁠, ⁠−1/2⁠ and ⁠1/−2⁠ all represent the same fraction – negative one-half. And because a negative divided by a negative produces a positive, ⁠−1/−2⁠ represents positive one-half.

In mathematics a rational number is a number that can be represented by a fraction of the form ⁠a/b⁠, where a and b are integers and b is not zero; the set of all rational numbers is commonly represented by the symbol ⁠${\displaystyle \mathbb {Q} }$⁠ or Q, which stands for quotient. The term fraction and the notation ⁠a/b⁠ can also be used for mathematical expressions that do not represent a rational number (for example ${\displaystyle \textstyle {\frac {\sqrt {2}}{2}}}$), or even do not represent any number (for example the rational fraction ${\displaystyle \textstyle {\frac {1}{x}}}$).

In a fraction, the number of equal parts being described is the numerator (from Latin: numerātor, "counter" or "numberer"), and the type or variety of the parts is the denominator (from Latin: dēnōminātor, "thing that names or designates"). As an example, the fraction ⁠8/5⁠ amounts to eight parts, each of which is of the type named fifth. In terms of division, the numerator corresponds to the dividend, and the denominator corresponds to the divisor.

Informally, the numerator and denominator may be distinguished by placement alone, but in formal contexts they are usually separated by a fraction bar. The fraction bar may be horizontal (as in ⁠1/3⁠), oblique (as in 2/5), or diagonal (as in 4⁄9). These marks are respectively known as the horizontal bar; the virgule, slash (US), or stroke (UK); and the fraction bar, solidus, or fraction slash. In typography, fractions stacked vertically are also known as en or nut fractions, and diagonal ones as em or mutton fractions, based on whether a fraction with a single-digit numerator and denominator occupies the proportion of a narrow en square, or a wider em square. In traditional typefounding, a piece of type bearing a complete fraction (e.g. ⁠1/2⁠) was known as a case fraction, while those representing only parts of fractions were called piece fractions.

The denominators of English fractions are generally expressed as ordinal numbers, in the plural if the numerator is not 1. (For example, ⁠2/5⁠ and ⁠3/5⁠ are both read as a number of fifths.) Exceptions include the denominator 2, which is always read half or halves, the denominator 4, which may be alternatively expressed as quarter/quarters or as fourth/fourths, and the denominator 100, which may be alternatively expressed as hundredth/hundredths or percent.

When the denominator is 1, it may be expressed in terms of wholes but is more commonly ignored, with the numerator read out as a whole number. For example, ⁠3/1⁠ may be described as three wholes, or simply as three. When the numerator is 1, it may be omitted (as in a tenth or each quarter).