Commas after introductory phrases: how short is short?

There are many rules in grammar that are matters of opinion: there is no right answer or wrong answer; instead, two different, sometimes contradictory, solutions are possible. The use of a comma after a short introductory phrase is one of those: you don’t have to use one (or not use one); it’s up to you.

Of course, there is a rule of thumb: the consensus is that you don’t necessarily need a comma after a short introductory phrase, unless it is a phrase of time, or a group of words that tell us when something is happening (in technical terms, an adverbial phrase of time). The trouble is that there is no corresponding rule of thumb about what constitutes short.  Just after the turn of the millennium, when I was rewriting Lonely Planet’s in-house style guide, I had a lengthy discussion with the company’s managing editors to see if we could define short for the purposes of all Lonely Planet books: three words? four words? We couldn’t agree, and there was no word limit specified in the style guide. We probably would have agreed that just after the turn of the millennium – at seven words – was not short and deserved a comma after it. Similarly, if it had functioned as the start of a sentence rather than a stand-alone phrase, the famous Star Wars opening – A long time ago in a galaxy far, far away  (ten words) – would have had a comma after it. (That opening phrase was followed by an ellipsis: … )

How short is short?

The purpose of a comma after these long opening phrases is the same as the main purpose of a comma: to show which words belong together. Generally an introductory phrase at the start of the sentence describes when (time), where (place), how (manner), why (reason) or to what extent (degree) the action of the sentence will happen.

  • Every Monday morning, I post a grammar-related blog on my website. (time – three words)
  • In a faraway kingdom, a princess woke up one morning. (place – four words)
  • With the greatest of care, the baker decorated the birthday cakes. (manner – five words)
  • For a future career in medicine, the student knew she needed high marks. (reason – six words)
  • To the utmost limits of my ability, I tried to complete the marathon swim. (degree – seven words)

Which of the phrases is short? Certainly the first two – maybe the third one? In some ways [three-word phrase], it doesn’t matter: as the comma is optional after a short introductory phrase, except one of time, you can have a comma or not as you please. Once you have a longer phrase, or the phrase is more about where, how, why or to what extent, then you do need the comma. Trying to determine the exact number of words is a bit like being a medieval theologian trying to decide how many angels fit on the head of a pin …

Phrases of time earn their special mention because they often contain numbers. As we usually use numbers to describe something else, a comma is often required to show which group of words a number belongs with: the words that follow it or the words that precede it.

For example: it doesn’t matter whether I write On Fridays, I do my weekly shopping at the supermarket or On Fridays I do my weekly shopping at the supermarket. The meaning is clear in either case. But consider these two sentences:

  • In March 2000 soldiers crossed the border.
  • From the beginning of the 18th dynasty kings were buried in tombs cut into the rocky cliffs within the Valley of the Kings.

Commas are absolutely necessary in the first example to make the meaning clear: was it 2000 soldiers crossing the border in March, or did an unknown number of soldiers cross the border in March 2000? The second example is clearer, but without a comma after dynasty, readers are likely to read the sentence as being about 18th-dynasty kings and misread it.

In a nutshell: if you have a long scene-setting description at the start of your sentence, use a comma (particularly if you would stop for a breath at the end of it, if you were reading it aloud). If you have a phrase that involves a number or numerical description (such as 18th dynasty), use a comma at the end of the number or numerical description. And if you have a short opening phrase (just two or three words, or maybe four or five) without a number, please yourself.  As always, use the comma to group your words together when it’s necessary to clarify meaning.

For 25 December, I wish you all a merry Christmas.

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