The MHRA Style Guide Online
A Handbook for Authors and Editors • Third Edition
From Chapter 5, 'Punctuation'
Commas are used singly or in pairs to indicate significant groupings or separations of words in the sentence. Particular note should be taken of the following usages:
(a) Commas should be used to delimit parenthetical or interpolated phrases, and nouns in apposition:
My father, not to mention the rest of my family, felt the loss deeply.
This book, written in 1505, would change the world.
Dante, the Florentine poet, was born in 1265.
Note that a noun preceded by a defining phrase is not in apposition and should not be enclosed in commas:
The Florentine poet Dante was born in 1265.
(b) Commas are used to delimit non-restrictive relative clauses, which do not define what precedes:
Those with a university degree, who have experience of higher education, see qualifications in a different light.
The family had two cats, which slept indoors, and a dog.
No commas are used in the corresponding restrictive relative clauses:
Those with a university degree who have studied medicine see research in a different light.
The family had two cats which slept indoors and one which went out at night.
(c) In an enumeration of three or more items, the practice in MHRA journals is to insert commas after all but the last item, to give equal weight to each enumerated element:
The University has departments of French, German, Spanish, and Portuguese within its Faculty of Arts.
The conjunctions and and or without a preceding comma are understood as linking the parts of a single enumerated element:
The University has departments of French, German, Spanish and Portuguese, Czech and Polish, and Dutch.
Comedians such as Laurel and Hardy, Abbott and Costello, or Charlie Chaplin.
By the same principle a comma should be used before a phrase such as ‘and so on’ or ‘etc.’ at the end of an enumeration.
The comma after the penultimate item may be omitted in books published by the MHRA, as long as the sense is clear. Usage should be consistent, although titles and subtitles may constitute exceptions because of considerations of graphic design and/or sense.
(d) Commas should not be used if their omission leaves the meaning of the sentence unaffected. The mere fact that a sentence has a complex subject does not justify the use of a comma between the subject and verb. Accordingly a sentence such as
The team of editors responsible for the latest edition of the handbook has made a significant number of changes.
requires no comma following ‘handbook’.