The expression "in the inside" appears to be logical (because insides are closed spaces with boundaries) but the more common expression is "on the inside." What’s the reason behind this usage?

  • 3
    I think with prepositions in a language it's better to just learn them, rather than going into the logic behind why they are that way. Commented Jul 29, 2019 at 18:29
  • 14
    The "logical" expression for "in the inside" is just "inside".
    – alephzero
    Commented Jul 29, 2019 at 18:58
  • 1
    'On' refers to a surface (area), and 'In' refers to a volume, and volumes have surfaces.
    – AmI
    Commented Jul 30, 2019 at 9:13
  • 2
    Note that it is also “on the outside” and not “out the outside”. Commented Jul 30, 2019 at 13:11

3 Answers 3


The noun inside, as opposed to the preposition inside, appears to be composed of a noun side, pre-modified by the preposition in.

The choice of preposition is driven by the nominal side morpheme, with which we normally use the preposition on. Metaphorically, the inside and the outside are the two surfaces either side of a dividing line, rather than two contained areas.


  • On the side
  • On the left side
  • On the right side
  • On the upside ...
  • On the downside ...
  • On the inside ...
  • On the outside ...
  • 14
    I don't think anyone actually says "on the upside" or "on the downside" when talking about actual physical "sides" (they'd say "on the top" or "on the bottom"), but yes, for metaphorical upsides and downsides (pros and cons), it follows the same pattern. Commented Jul 29, 2019 at 16:43
  • 3
    @ShadowRanger For physical locations we'd usually say "on the upper side" and "on the lower side," but those are exactly parallel to the "left" and "right" examples above.
    – cjs
    Commented Jul 30, 2019 at 7:15
  • 9
    'on the flipside'. Now it's complete.
    – Mitch
    Commented Jul 30, 2019 at 18:05
  • This is a very high quality answer. Thank you for expanding my mind.
    – Fomalhaut
    Commented May 10, 2023 at 16:49

If you wanted to express that a fly was crawling on the inside surface of the sphere of a football, you would say :

It is crawling on the inside of the ball.

But if you wanted to express that the fly was flying inside the empty sphere of the ball you would say :

It is flying inside the ball.

'Inside' already means 'within'. So to say 'in the inside' is duplicating meaning.

  • 2
    This is the most clearly direct answer to the question of why we don't say in the inside. Commented Jul 29, 2019 at 18:04
  • In the inside of a tootsie roll pop is a sphere of tootsie roll. Commented Jul 29, 2019 at 21:40
  • @WayfaringStranger Inside a tootsie roll pop . . . etc.
    – Nigel J
    Commented Jul 29, 2019 at 23:35

As Araucaria said, we usually speak of something being "on the side" of something.

Sometimes the prepositional phrase "on the inside" describes something that is literally on a surface. For example, consider a pipe: I can say "there's rust on the inside" to express that the inside surface of the pipe has rust on it. If I instead wanted to talk about the contents of the pipe, "inside" could be used by itself, without any additional preposition before it: "there's water inside the pipe", or the preposition in could be used: "there's water in the pipe".

The word on does have a broader sense and does not just refer to things that are directly touching a surface. But the basic sense of on the inside can often be seen as referring to something's position relative to the internal surface of some object, as suggested by the Merriam-Webster entry for the expression (cited in user067531's now-deleted answer):

1 : on the inner side, edge, or surface of something

  • The number 22 car tried to pass the leader on the inside (of the track).

So it may be that "in the inside" isn't used because "in" and "inside" are shorter ways to express the idea of something being in the space enclosed by a boundary.

  • Perfect. . . . .
    – Andrew
    Commented Jul 30, 2019 at 13:47
  • On the bright side but in the shadow.
    – choster
    Commented Jul 30, 2019 at 16:29

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