<P> In mathematics the set of all numbers that can be expressed in the form a / b, where a and b are integers and b is not zero, is called the set of rational numbers and is represented by the symbol Q, which stands for quotient . The test for a number being a rational number is that it can be written in that form (i.e., as a common fraction). However, the word fraction is also used to describe mathematical expressions that are not rational numbers, for example algebraic fractions (quotients of algebraic expressions), and expressions that contain irrational numbers, such as √ 2 / 2 (see square root of 2) and π / 4 (see proof that π is irrational). </P> <P> 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 ​ ⁄ 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 . </P> <P> Informally, the numerator and denominator may be distinguished by placement alone but in formal contexts they are always separated by a fraction bar . The fraction bar may be horizontal (as in 1 / 3), oblique (as in 1 / 5), or diagonal (as in ​ ⁄). These marks are respectively known as the horizontal bar, the slash (US) or stroke (UK), the division slash, and the fraction slash . In typography, horizontal fractions are also known as "en" or "nut fractions" and diagonal fractions as "em fractions", based on the width of a line they take up . </P> <P> The denominators of English fractions are generally expressed as ordinal numbers, in the plural if the numerator is not one . (For example, ​ ⁄ and ​ ⁄ 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 as simply "three".) When the numerator is one, it may be omitted . (For example, "a tenth" or "each quarter".) </P>

What do you call the line in a fraction