The diagonal slash "/" used as the bar between numerator and denominator of an in-line fraction (Bringhurst 1997, p. 284). The solidus is also called a diagonal.

Special care is needed when interpreting the meaning of a solidus in in-line math because of the notational ambiguity in expressions such as . Whereas in many textbooks, "" is intended to denote , taken literally or evaluated in a symbolic mathematics program such as Mathematica, it means . For clarity, parentheses should therefore always be used when delineating compound denominators.

Common examples of failure to parenthesize include (where is energy, is Boltzmann's constant, and is temperature; Arfken 1985, p. 950), its variant (where is a constant and is wavelength; Weast 1981, pp. F-109 and F-111), the exponent in the normal distribution (where is the standard deviation; Hastings 2000, p. 217). Other miscellaneous examples occur even in the standard references for computer math systems (e.g., Wolfram 2003, pp. 776, 779, and 787).