Extension vs Supplement

When to use each in English, with meaning, register, and examples.

Extension

Top 2,000 (common)B2noun

Supplement

Beyond 10,000 (less common)C1noun
Most common: Extension
 ExtensionSupplement
Pronunciation🇬🇧 /["/ɪkˈstenʃn/"]/🇺🇸 /["/ɪkˈstenʃn/"]/🇬🇧 /["/ˈsʌplɪmənt/"]/🇺🇸 /["/ˈsʌplɪmənt/"]/
MeaningAn addition to something, like a part that helps it work better.Something added to help or improve something else.
ExampleThe deadline for the project got an extension of two weeks.The textbook serves as a supplement to the main course material.
RegisterNeutralNeutral
How commonTop 2,000 (common)Beyond 10,000 (less common)
CEFR levelB2C1
Part of speechnounnoun
Collocationsconsiderable, great, major, planned, proposed, home, add, build, extension to, planned, proposed, home, add, build, extension to, one-week, two-year, etc., apply for, ask for, request, planned, proposed, home, add, build, extension to, considerable, great, major, planned, proposed, home, add, build, extension touseful, valuable, colour/​color, provide (somebody/​something with), add, take, supplement to, useful, valuable, colour/​color, provide (somebody/​something with), add, take, supplement to, useful, valuable, colour/​color, provide (somebody/​something with), add, take, supplement to, additional, extra, $100, add, charge, carry, be payable, at a supplement, supplement for, supplement on
Antonymsrestriction, limitationreduction, diminution, lessening
Common mistakesConfused with 'extent' — make sure to use the right context., Using 'extension' when 'addition' is more appropriate., Mispronouncing it as 'extention' instead of 'extension'.Confused with 'complement', which means something that completes another., Using it as a verb when referring to physical items instead of additional support.
Usage notesCommonly used in business and technology contexts. Avoid using it in very casual conversations. It's appropriate when discussing things like phone numbers or deadlines.Used in contexts related to health, nutrition, and finance. Avoid in casual conversations unless discussing specific dietary needs or financial advice.

Frequently asked questions: Extension vs Supplement

What's the difference between Extension and Supplement?

Extension: An addition to something, like a part that helps it work better. Supplement: Something added to help or improve something else.

Which is more common: Extension and Supplement?

Extension is the most common in everyday English.

Are Extension and Supplement the same CEFR level?

Extension: B2, Supplement: C1 on the CEFR scale.

Can I use Extension and Supplement interchangeably?

Not always. Extension and Supplement are related and overlap in some contexts, but they differ in register, how common they are, and usage, so swapping one for another can change the meaning or tone. Check the differences above before substituting.

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