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Punctuation. Commas

Interuniversity Style Guide

    The content of these guidelines is taken from the third edition of the Vives University Network’s Interuniversity Style Guide for Writing Institutional Texts, an interuniversity project in which the UPC participates with the support of the Secretariat for Universities and Research of the Government of Catalonia.


      General guidelines

      • As a general rule, commas can be used to list items in a series, to join sentences and to set off parenthetic or introductory phrases
      • For commas in lists in sentence form, see Elements in lists.


      Listing items in a series

      • In a list containing a series of items, separate the items with serial commas. However, a serial comma should not precede the conjunction before the final item (in other words, write a, b and c and not a, b, and c). But if a comma would make the meaning clearer, use it – especially where one of the items in the list is already joined by and.

        Specialist subjects include teaching, research and development, and business applications.


      Joining sentences

      • When you join two complete sentences into a single sentence, you can use commas but follow them with a suitable connecting word: and, or, but, while or yet.

        The group members had to hand in their reports last week, but some were only submitted this week.

      • The comma is not required if the subject of the second part of the sentence is omitted or if the conjunction used is and or or.

        The student had to hand in the work by Friday but didn't make the deadline.
        The student had to hand in the work by Friday or the work would receive a failing mark.


      Setting off phrases

      • If a phrase is meant to complement or introduce the main information in a sentence, it can be set off by a comma or pair of commas.

        Knowing that some students put things off, the lecturer decided assignments should be submitted a week before the exam.
        The students in the third group, despite their best efforts, did not submit the assignment on time.


      Setting off phrases: defining relative clauses

      • In relative clauses do not use commas if the clause defines the antecedent and if omitting it would radically change the meaning of the sentence. The clauses below define the type of students referred to.

        Students who study here are very intelligent.
        Students who use the library are likely to be better prepared.

      • In defining clauses that do not refer to people use either which or that. The clauses below define the type of materials referred to.

        The research was conducted with materials which were easily obtained.
        The research was conducted with materials that were easily obtained.


      Setting off phrases: non-defining relative clauses

      • Sentences with non-defining relative clauses need commas because the clause adds information to an otherwise complete sentence. In non-defining clauses, only use which or who.

        Students at that university, who were all admitted with academic scholarships, have to work very hard.
        The research, which was done over a period of three years, was conducted with easily obtained materials.

      • In the following two sentences, which are both correct, the comma makes a difference. In the first, Dr Smith researches only those additives that pose a risk; in the second, the implication is that all food additives pose a risk.

        Dr Smith researches food additives which pose a risk to human health.
        Dr Smith researches food additives, which pose a risk to human health.


      Other uses of commas

      • Separate a city from a state, province, region or country with a comma.

        Chicago, Illinois Lisbon, Portugal

      • Do not use a comma between the month and the year.

        October, 2001 October 2001

      • In most numbers of one thousand or more, use commas between groups of three digits, counting from the right.

        62,242 1,723 1,000,000

      • Exceptions are degree temperatures, year numbers of four digits, addresses, page numbers and other uses of numbers for a non-quantifying purpose (see Numbers and commas).