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Prepositions before nouns

“Of law” or “by law”?

These expressions have different meanings and grammar. One shows relation to the subject of law, while the other refers to legal authority.

Quick answer

Using “of law” and “by law”

Use by law to mean required or established legally. Use of law in phrases that describe relation to the field or concept of law, usually with another noun.

Smoking is prohibited by law
the principles of law

Choose the pattern by meaning

These forms are not interchangeable. Choose the construction by meaning, not by which preposition appears most often.

1
by + law

Legal requirement or authority

Use by law for something required, permitted, or defined legally.

Employers must provide breaks by law
2
of + law

Relation to the legal field

Use of law after another noun to show connection to law.

a professor of law

See the difference in context

The same subject can take different constructions because each sentence answers a different question.

Construction
Example
Meaning
of law
the history of law
Shows relation to the subject or system of law.
by law
Helmets are required by law.
Means legally required or according to legislation.
under the law
Everyone is equal under the law.
Refers to legal systems or rules.

Common mistakes

Some combinations appear in real text but do not mean what learners often assume.

Writing bylaw instead of by law

Bylaw is a noun meaning a local rule. By law is a prepositional phrase.

✕ It is required bylaw. ✓ It is required by law.

Using of law to mean legally required

Of law does not express legal obligation.

✕ Seat belts are mandatory of law. ✓ Seat belts are mandatory by law.

Quick practice

Choose the expression that matches the intended meaning.

1. Drivers must stop here ___ law.
2. She teaches the philosophy ___ law.
3. All citizens are protected ___ the law.

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Frequently asked questions

What does “by law” mean?

It means according to legal rules or legislation.

Can “of law” stand alone?

Usually it follows another noun, such as principles of law.

Is “bylaw” the same as “by law”?

No. A bylaw is a local regulation. By law is a phrase meaning legally.

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