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Prepositions with extension

By extension, of extension, or for extension?

By extension is a fixed phrase meaning “as a further result or consequence.” Of extension and for extension are possible only in specific noun phrases, not as general alternatives.

Quick answer

Which form is correct?

Use by extension to introduce a related conclusion or broader application. Of extension usually appears after another noun, such as degree of extension. For extension is used when something is intended for extending or for an extension period.

by extension, the new policy affects contractors too.
The doctor measured the extension of the patient’s knee.
We applied for an extension of the deadline.

Choose the pattern by meaning

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

1
by extension

Showing a further consequence or broader meaning

Use by extension to connect one idea to a related conclusion.

By extension, the rule applies to remote workers as well.
The tax affects imports and, by extension, local prices.
2
extension of + noun

Describing what is extended

Use extension of when naming the thing being stretched, continued, or enlarged.

The extension of the contract was approved.
Researchers studied the extension of the glacier over time.
3
for an extension

Requesting extra time or continuation

Use for with extension when asking or applying for more time.

She asked for an extension on the essay.
Students may apply for an extension before Friday.

See the difference in context

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

Construction
Example
Meaning
by extension
By extension, the decision could affect suppliers.
a related consequence or broader application
of extension
The report included measurements of extension and flexion.
part of a technical noun phrase, especially in medicine or mechanics
for extension
They submitted a request for an extension.
used with requests or purposes, not as a replacement for by extension
extension of
The extension of the visa took two weeks.
continuation or enlargement of something

Common mistakes

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

Using of extension as a general connector

Of extension is not a standard phrase for introducing a conclusion.

✕ Of extension, the policy affects freelancers. ✓ By extension, the policy affects freelancers.

Using for extension without an article

Extension usually needs an article in this meaning.

✕ She applied for extension. ✓ She applied for an extension.

Confusing extension of with by extension

These expressions have different functions.

✕ Extension of, the problem spread further. ✓ By extension, the problem spread further.

Quick practice

Choose the expression that matches the intended meaning.

1. ___, the safety rule also applies to visitors.
2. He submitted a request ___ the deadline.
3. The therapist measured the ___ the elbow.

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Frequently asked questions

Is by extension a fixed phrase?

Yes. It is a standard expression meaning “as a further result” or “in a broader sense.”

Can I say of extension by itself?

Usually no. It normally appears inside a larger noun phrase such as range of extension or extension of the arm.

When do we use for an extension?

Use it when asking for more time, continuation, or expansion.

"behind bar" or "of bar"? "beyond description" or "of description"? by, accident, of, accident "by boat" or "of boat"? "by chance" or "of chance"? "by default", "in default" or "of default"? "by lot" or "with lot"? by, of, for, in or agreed-upon definition? by, of, in, into or to category? by, of, in, via or on telephone? by, of, on, for or in bus? by, of, to, with or from professor?

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