1. The Commonwealth Edison Company, which controls the electric light and power supply of Chicago, was organized in 1907 by the consolidation of the Chicago Edison Company and the Commonwealth Electric Company.
The sentence makes complete sense without the relative clause.
2. The concern that controls the electric light and 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?
1. The man wearing the brown coat is my brother.