Q. When is it proper to use an ampersand? Thank you.
A. In edited prose, use of the ampersand—&*—is normally limited to
- terms like R&D and Q&A that are always spelled with an ampersand (see also CMOS 10.10);
- corporate names like AT&T and Simon & Schuster that reflect the usage of a particular company or brand (see also CMOS 10.24); and
- ampersands in verbatim quotations.
An ampersand may also be used when mentioning the title of a work that includes one (subject to editorial discretion; see CMOS 8.165). And if you’re working with HTML, you may need an ampersand in a character reference like (for a nonbreaking space)—or & (for an ampersand).
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* According to the OED (and other sources), the ampersand evolved from a stylized rendering of the Latin conjunction et (and) in the form of a ligature, and the word ampersand is an English-language corruption of “& (and) per se and” (“and by itself [is] and”)—a phrase that differentiated the symbol from &c., an old-fashioned way of writing et cetera (and others).
[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]
Q. I have an author who is hot under the collar because we replaced all of the ampersands in company names with “and,” per CMOS (14th ed.). The author insists that this is incorrect and that the ampersand is part of the legal name of the company. Can you help me?
A. Your interpretation of the fourteenth edition is correct: CMOS 14 regards ampersands as abbreviations that may be changed to “and” in running text. Exceptions include expressions like “R&D” and names of corporations that are generally abbreviated, such as AT&T, where it would be odd to spell out “and” but not the rest of the abbreviation.
[A couple of decades and a few administrations later:] The seventeenth edition would side with your author (see paragraph 10.24). If a company appears to prefer an ampersand in its name, then write it that way (e.g., Marks & Spencer). If in doubt, you can spell it out. Be consistent.
[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]