- By late afternoon, the day's heat had peaked.
- She decided to shop at Grey's Market, though it was out of her way.
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.
- In 1982, something happened.
- Under the table, people find things that have fallen.
- Finally, the thing ended.
- Surprisingly, something else happened.
- If something is like this, another thing will happen.
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.
- Students who don't do the reading are unlikely to pass the exam.
- Marcy, who always read the assigned pages, received an A on the exam.
Similarly, other "aside" phrases or comments (‘of course,’ ‘indeed,’ ‘i.e,’ ‘eg.,’ etc.) that can be considered optional are also set off with commas.