The Comma
- 1 Using a comma to separate two or more adjectives
- 2 Commas in a simple series
- 3 Commas and independent clauses
- 4 Commas and introductory dependent clauses
- 5 Commas and introductory elements
- 6 Using commas with dates
- 7 Commas and titles
- 8 Commas and Geographic Names and Addresses
- 9 Commas in units of measure
- 10 Commas in drug dosages and instructions
- 11 Commas in laboratory values
- 12 Commas in dialogue
- 13 Commas to indicate omitted words
- 14 Commas in nonessential expressions
- 15 Commas and interrupting elements
- 16 Commas used with appositive expressions
- 17 Commas to set off afterthoughts
- 18 Commas and transitional expressions
- 19 Genetics and chromosome punctuation
The comma has two primary functions: it sets off nonessential expressions that interrupt the flow of thought from subject to verb to object or complement, and it separates elements within a sentence to clarify their relationship to one another. Two commas are typically needed to set off, but only a single comma is needed to separate.
As with any aspect of grammar and punctuation, an understanding of sentence structure is necessary.
Basic Rules for Commas that Separate
Use a single comma to separate the two main clauses in a compound sentence when they are joined by and, but, or, or nor.
Use a single comma to separate three or more items in a series unless all the items arejoined by and or or.
Use a single comma to separate two or more adjectives that modify the same noun.
Basic Rules for Commas that Set Off
Use commas to set off nonessential expressions; words, phrases and clauses that are not necessary for the meaning or the structural completeness of the sentence.
In many sentences, you can tell whether an expression is nonessential or essential by trying to omit the expression. If you can leave it out without affecting the meaning or the structural completeness of the sentence, the expression is nonessential and should be set off by commas.
Nonessential: We will consult with Dr. Smith, who has expertise in neurological disorders in neonates.
Essential: Let's get a consult with someone who has expertise in neurological disorders in neonates.
Nonessential: There is, no doubt, a reasonable explanation for his elevated LFTs.
Essential: There is no doubt his elevated LFTs can be reasonably explained.
More complex comma use is outlined in the detailed pages shown in the Table of Contents above.
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