The final rule for using a comma is when separating information which is not essential to the grammar of the sentence. For example: Public transport, such as trains and buses, is often rejected in favour of private modes of transport. If we remove the section in commas, the sentence is still OK: Public transport is often rejected in favour of private modes of transport. This means that it is not required to make an accurate sentence. However, we do not use commas if the information is required for the grammar of the sentence. For example: The man who lives upstairs is a doctor. If we remove who lives upstairs, the sentence is not grammatically correct because we do not know who ‘the’ man is.