I am looking for an word (adjective or noun) that describes the fact of two (or more) objects to be the same externally but different internally.

For example, how would you characterize the quality of two devices that perform the same function yet are intrinsically different when you examine them closer?

I am thinking of the word 'twin', but I am not completely satisfied with it. Any ideas?

Example sentence:

This mechanical system can be in two different states, yet undistinguishable for an external observer. One can cause the system to switch between these _______ states by applying a certain force...

  • Can you give the sentence where you would use the word?
    – fev
    Nov 15 at 14:05
  • @fev Example sentence: 'This mechanical system can be in two different states, yet undistinguishable for an external observer. One can cause the system to switch between these [insert sought adjective] states by applying a certain force...'
    – PDC
    Nov 15 at 14:15
  • 1
    Not a single word: they are superficially the same. Nov 15 at 14:17
  • Since the example sentence already makes the point, you could say these alternate states. Nov 15 at 14:18
  • 1
    In many technical contexts we might refer to any given implementation as a "black box" - where we don't know or care what's going on inside, so long as this black box performs exactly the same function as that black box. Nov 15 at 15:24

4 Answers 4


You could say that the two states are (like) chalk and cheese.

Here's Wiktionary defining the idiom:

Said of things that very different, though possibly superficially alike.


Attested in variant form since 1393. The surface of some cheeses, particularly unaged ones, looks similar to chalk – white, crumbling – but in substance cheese is much softer than chalk


This won't be a popular answer, but I'm going to say that there is probably not a word for this. It isn't a common situation, and where we do have a situation where two things look the same but are actually different, and we need to talk about them, we are likely to invent language for it (just as how quantum physicists invented nonsense like "red", "blue" and "strange" merely to differentiate things that aren't easy to tell apart in reality).

In a real conversation in English, you'd probably use a phrase like "only superficially alike".


Not a very academic term and it may not work in your context; kind of a hodgepodge to be honest, but:

you could preface your idea of 'twins' with 'illusory,' thus making 'illusory twins.' (or even 'illusive twins,' depending on your intent here).

I would recommend that you keep the word 'twins' but prefix it with something like fallacious, spurious, deceptive, faux, or any such word that carries a similar connotation.

I hope that this helps!


Look alikes: Freedictionary: 1. noun A person or thing that looks very similar to another person or thing. When used as a noun, the term is often hyphenated or spelled as one word. 2. verb To look very similar to another person or thing.

  • This doesn't answer the question. A lookalike is the same on the outside, but it might be the same on the inside too. The question asks for something that's the same on the outside but DIFFERENT inside.
    – equin0x80
    Nov 19 at 11:48
  • It does answer the question. Identical means same externally and internally, look alike means "only looks are alike" which is external. If both external and internal were alike "identical"would be used, not "look alike". Kindly remove the negative vote.
    – Wolfim
    Nov 20 at 14:06

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