The prime symbol ′, double prime symbol ″, triple prime symbol ‴, and quadruple prime symbol ⁗ are used to designate units and for other purposes in mathematics, science, linguistics and music. Although the characters differ little in appearance from those of the apostrophe (or the single or double quotation mark), the appropriate uses of the prime symbol are different. While an apostrophe is often used in place of the prime (and a double quote in place of the double prime) due to the lack of the prime and double prime symbols on everyday keyboards, such substitutions would not normally be considered appropriate in formal materials or in typesetting.