The comma is the most-used punctuation mark in English and, used properly, it will make your writing much easier to read and understand.

Straightaway, let’s look at the previous sentence. I have put commas before and after ‘used properly’ as they form a non-restrictive relative clause. What the xxxx! I’m sorry, you really don’t need to know fancy terms like that – I was just showing off. A non-restrictive relative clause is just extra information, and if you take this extra information out of the sentence the basic meaning won’t be changed. So if I take ‘used properly’ out of the sentence, the meaning is pretty much the same.

Removing a restrictive relative clause will change the meaning of a sentence, like this:
Restrictive (qualifying information):
The museum which is in the High Street is dedicated to natural history.

This sentence means that there is more than one museum, and that the one I’m talking about is in the High Street.
Non-restrictive (extra information):
The museum, which is in the High Street, is dedicated to natural history.
This sentence means that there is only one museum in the area, and it happens to be in the High Street.

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