Relative clauses （関係節）with commas and without commas

There are two types of relative clauses:
TYPE A
The relative clause is not essential to the sentence. Even if it is omitted, the sentence can be understood. This type of relative clause is separated from the rest of the sentence through the use of commas. It often gives a reason. For example, in the first sentence below, the implication is that Keio's prestige stems at least partly from its long history.

Keio, which was founded in 1858, is one of Japan's most prestigious educational institutions.
The Japanese ￥10,000 note has a picture of Fukuzawa Yukichi, who played an important role in introducing Western ideas to Meiji Japan.
The country has only one airport, which is used mainly for domestic flights.

TYPE B
The relative clause is essential to the sentence. If it is omitted, the sentence may have little or no meaning (as in the first two examples), or the meaning will change (as in the last example). This type of relative clause is never separated from the rest of the sentence through the use of commas.

The countries that most influenced the development of Meiji Japan are probably Britain and Germany.
Children need a person whom they can both love and respect.
The country has only one airport which is used mainly for domestic flights.

Note the difference in meaning between the last examples of both types A and B:
TYPE A: The country has only one airport, which is used mainly for domestic flights.
Even if the relative clause is omitted, the sentence can still be understood. In other words, this country has only one airport.
TYPE B: The country has only one airport which is used mainly for domestic flights.
Without the relative clause, the sentence would not be understood. In other words, the country has several airports. One of them is mainly used for domestic flights, but the others are mainly used for international flights.

Try this quiz, by Jill Cosh, Anglia Ruskin University