Can anyone please help me to understand the difference between the 2 words? I for one understand that the words have a similar meaning but when is it correct to use them? I use "extendable" for when it's about physical objects such as couch, a spyglass or the pipe that I have at my vacuum cleaner, things like that, that can be folded and unfolded.

What about "extensible" though? I rarely use the latter in conversation and writing and I want to clarify for some friends because to my understanding, does it refer to add parts to an object, in a way, expanding it? Is it a good synonym with expansion? Thank you and I apologize if this was discussed before but I would like some good examples to know in what cases these words should be used.


I think of extensible as "able to be added to with minimal disruption or reorganization".

You're not the only one who rarely uses the word. I don't think it's very common outside of some technical contexts.

One place I've seen it used is software design. An extensible architecture is one where a program is designed such that new functionality can be added without too much restructuringof the existing code. So someone might say:

Using more specialized classes will make this software more extensible.

(This is discussed further in the Wikipedia article on Extensibility.)

  • It can also be used for things that can be "made longer". For example, Hummingbirds have a long, slender bill with a long extensible tongue adapted for nectar feeding.. – ColleenV Jun 6 '17 at 11:28
  • Isn't similar to my examples? For example, I have a couch and I can "make it longer" and turn it into a bed and I would call this an extendable couch. If you go to a furniture store for example, what term do you mostly encounter? I am not a native English speaker but I think that "extendable" is more commonly used, even though I only heard it in TV shows/movies and for that reason I am used to using it. – Alice B. Rabbit Jun 6 '17 at 11:48
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    @Alice I wonder if the difference is that extensible in the sense of "protruding" is biological/medical, and extendable (in tge sense of make longer, not add to) is for objects like those in your examples. I will see if I can verify that guess when I have more time. – ColleenV Jun 6 '17 at 13:46
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    I think there is probably a lot overlap between the two words, so it will be difficult to support a case if you want to say, "These usages are okay, but these are not." One word may sound more normal than another, but correctness is rarely so cut-and-dried in English. – J.R. Jun 6 '17 at 14:56
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    My sense of it is that "extendable" refers to something that can be expanded in a specific, pre-designed way. "Extensible" refers to a generic capability of lending itself to be easily expanded in previously undefined ways. – fixer1234 Jun 6 '17 at 20:41

An analogy I use to illustrate the difference is this:

  • The collapsible, Slinky-like, pull-to-make-longer hose on your fuel can / jerry can is inherently extensible. It can be made longer with no add-on modifications.

  • The hose on your Shop-Vac or vacuum cleaner can be made functionally longer by snapping on additional lengths of hose. It is extendable.

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