power supply of Chicago_ is the Commonwealth Edison
Company.
The relative clause must be used to understand the sentence.
In (1) the relative clause gives an additional idea. In (2) it limits or
restricts the meaning of _the concern_. The non-restrictive clause is
shown in (1), the restrictive clause in (2).
Dictation to illustrate non-restrictive clauses:
It is estimated that Chicago annually uses 93,450,000
gallons of milk, for which it pays over $28,000,000.
To supply this amount 120,000 cows are needed, which
are owned by 12,000 dairy farms. Health officers
conduct a systematic dairy farm inspection, which has
for its purpose the exclusion of diseased milk. Farm
owners, who formerly objected to the inspection, now
see that cleanliness is profitable. Authorities have
discovered that milk, which easily absorbs germs, is
dangerous except when produced under sanitary
conditions, and now dairies are allowed to sell only
clean, pure milk, which is milk given by a healthy
cow.
Phrases as well as clauses may be restrictive. In the following
sentences decide whether the italicized expressions are restrictive or
non-restrictive. State whether they are phrases or clauses. Do any of
the sentences need commas?