That and Which
It can be difficult to know when to use that and which. It depends on whether you are introducing information that is essential, or extra information.
Giving Essential Information
We use that when the information is important, or the sentence wouldn’t make sense without it.
This is the wedding dress that my grandmother made.The primary information is that my grandmother made the wedding dress. This information is essential, because the sentence would lose its meaning without it.
It is also acceptable to use which for important information. It has the same meaning as that. However, many people prefer to use that for essential information.
When you use that or which for essential information, don’t use commas.
Giving Extra Information
We use which to provide extra information about something. The sentence should still make sense without the extra information.
The wedding dress, which my grandmother made, is on display in the museum.The primary information is that the wedding dress is on display in the museum. The fact my grandmother made the dress is extra information; the sentence is still complete without this information.
When we use which to provide extra information, you need a comma before which and a second comma after the extra information. Put commas at the beginning and end of the clause you could remove.
The wedding dress, which my grandmother made, is on display in the museum.Notice the commas before and after the extra information. We could write the sentence as ‘The wedding dress is on display in the museum’ and it would still make sense.