Hi, I have looked at this sentence for ages, but can't figure out how the commas have changed the meaning. Please help.
Cats who often go out at night make great pets.
Cats, who often go out a night, make great pets.
Thank you for any help.
(1) "Cats who often go out at night make great pets".
(2) "Cats, who often go out at night, make great pets".
The underlined clause in (1) is an integrated (or defining) relative clause, while in (2) the commas mark it as being a supplementary (or non-defining) relative clause.
The difference is that an integrated relative clause delimits the meaning of the head noun, in this case "cats". Ex (1) means that only cats who often go out at night make great pets. It implies that cats who don't often go out at night don't make good pets.
By contrast, a supplementary (or non-defining) relative clause provides non-essential information about the head "cats". Think of it as being a separate unit of information, parenthetical or additional, which can be omitted without changing the core meaning of the sentence. So Ex (2) means that all cats make great pets, and by the way they often go out at night.
Have I explained that clearly enough?