1

It's a device you put in your car, that allows you to find your destination.

It's a device you put in your car that allows you to find your destination.

I think both are ok, but I am not sure. Which one would you choose and is there any rule that dismiss one of them as ungrammatical, or are both correct? I might only add a comma for clarification, but I feel it might not be needed.

2

The comma is a bad choice here. Many style guides will advise using a comma before "which" but not before "that" when forming a relative clause. While this isn't an absolute rule it should be applied here. Changing "that" to ", which" should be considered as an option here, if you want the "non-restrictive" relative clause.

A comma with "That" has a further problem here: it would suggest a different reading. Consider:

John put the device in his car; that meant he could find his destination easily.

In this sentence, "that" refers to "putting the device in the car". It doesn't refer to the device. Placing a comma would tend to initially be read as referring to "putting", you would have to backtrack when the context makes it clear that the meaning is different.

In short, while it is not ungrammatical, but you should not use a comma before "that" in the context that you give.

  • You have a semicolon instead of a comma here, some people might actually use a comma when a semicolon would be more “correct” or “standard” (as I just did). In other words, I agree that a comma might be read as a semicolon in the OP’s sentence and would not convey the intended meaning. – Mixolydian Mar 10 at 1:29
1

This is difficult to answer, since you are actually combining two different elements of grammar.

In both UK and US English, a nonrestrictive clause uses which after a comma:

It's a device you put in your car, which allows you to find your destination.

In both UK and US English, that can be used in a restrictive clause:

It's a device you put in your car that allows you to find your destination.

However, in UK English which can also be used (in US English which can be used, but it's not nearly as stylistically common):

It's a device you put in your car which allows you to find your destination.


More generally speaking, is the clause after car restrictive or nonrestrictive? In other words, is the fact that it allows you to choose your destination essential information you have to convey—or is it merely additional information? Can the main point of the sentence stand on its own without anything after car?

If it's essential, it's restrictive (and there should be no comma); if it's additional, it's nonrestrictive and there should be a comma. And whether you use than or which will depend on what it is and where you live.

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