Which is correct:

A laser was used where the wavelength was 15 cm.


A laser was used, where the wavelength was 15 cm.

(Note the comma before where)

More generally, do we use a comma before "where" and "in which"?

"In which" example:

X-ray diffraction uncovered an in-plane texture, in which gold preferentially aligned with sapphire.


X-ray diffraction uncovered an in-plane texture in which gold preferentially aligned with sapphire.

  • In your "where" sentence, we would not insert a comma. Meanwhile, I would need to see a sentence in which "in which" were used to better help you, but on the whole, also no. Also, I would have said, "...was used when the wavelength...." Apr 10, 2017 at 11:46
  • In your second example, I think "preferentially" is not the right word. Or if it is, many of the other words need to change.
    – Mr Lister
    Apr 10, 2017 at 11:50
  • Since the second part 'gold preferentially aligned with sapphire' describes the 'X-ray diffraction uncovered an in-plane texture', a comma is appropriate. Apr 10, 2017 at 11:53
  • @MrLister is correct, maybe 'in which gold is preferentially aligned with sapphire' would be better? Apr 10, 2017 at 11:54
  • 3
    Arguable at worst, not wrong at best. Personally I would re-write the first to something like "A laser with 15 cm wavelength was used" but I wouldn't object to seeing it as you've written without the comma. I prefer the second sentence without a comma as well and think preferentially is perfectly fine as-is. But, I understand that not everyone is as comfortable with giving agency to things in this way.
    – MDHunter
    Apr 10, 2017 at 12:04

1 Answer 1


Use a comma before non-restrictive clauses, and don't use a comma before restrictive clauses. See this webpage.

How do you know whether a clause is restrictive or non-restrictive? It's restrictive if the information is essential, and limits the scope of the thing it's modifying. It's non-restrictive if the information is non-essential. For both of your sentences, the clause can be analyzed as either restrictive or non-restrictive, so in cases like this, the comma is optional.

  • 1
    A comma may be optional, but in the laser pair, the first means that in cases where the wavelength was 15cm a laser was used (and presumably something else in other cases), while the second means that a laser was used, and that the wavelength of that laser was 15cm.
    – mattdm
    Nov 26, 2017 at 21:08

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