Insert a comma when providing additional information that is secondary to the main point of a sentence. In grammar-speak, that is using commas to separate the main clause from subordinate and/or certain relative clauses.Examples can make this clearer:
In each of the foregoing sentences, some useful detail is lost by dropping the subordinate clause (before the comma in the first sentence and after the comma in the second), but the meaning of the sentence isn't affected.
In practice, this means that commas will usually follow certain introductory phrases at the beginning of a sentence, including indicators of time or sequence, place, extent, cause, degree or condition.
Relative clauses are ones that begin with ‘who,’ ‘which,’ ‘that,’ ‘whom,’ or ‘where.’ They can go either way. Apply the 'essential to meaning' test to see if you need to use commas or not. In this case, the phrase will be set off before and after with a pair of commas.
Similarly, other "aside" phrases or comments (‘of course,’ ‘indeed,’ ‘i.e,’ ‘eg.,’ etc.) that can be considered optional are also set off with commas.
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