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As a native English speaker (Australia) I've always known and used the expression "to know something inside out", meaning "to know thoroughly".

Just now when editing a post on another SE site that seemed to be written by a non-native speaker I was going to change "to know something inside and out" to "to know something inside out" but thought I'd check first.

I knew "inside and out" to have a different meaning and usage such as "The vehicle was fully restored inside and out".

I was surprised to find the English Wiktionary has a main entry under "to know something inside and out" and a stub entry only for "to know something inside a" declaring it to be a variant of the former!

This was contradicted by a Google search saying the version without "and" was a lot more popular, but a Google Ngram search seemed to suggest that the two had each gone up and down in popularity over time. So I decided this was all inconclusive.

So what do real/reliable/print dictionaries and other sources have to say about the matter? Are both considered OK, is one preferred, is one Euro-English that made it into the English Wiktionary, or has the expression changed over time, or does it vary by region?

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+1 I would be interested in the answer as well and to confirm: I am indeed a non-native speaker of English. – Relaxed Apr 22 '14 at 11:26
I would also use I know the subject outside in . Just to complicate matters. – Frank Apr 22 '14 at 12:20

I expect that the original phrase was inside and out. It refers to knowing all the parts of something (both its inner workings and its outside appearance).

However, inside out is a more common phrase, meaning an object whose insides and outsides have been reversed. Speakers may have shortened the above phrase to this one that they're more familiar with, even though it makes no literal sense for the above meaning.

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+1 Exactly. People think in chunks of words, and similar chunks get mixed up. The parade example is "I could care less." – StoneyB Apr 22 '14 at 12:03
I was also thinking of how many people turn all intents and purposes into all intensive purposes. – Barmar Apr 22 '14 at 12:07
Yes - though I would classify that as a Mondegreen. – StoneyB Apr 22 '14 at 12:16
Possibly, although it seems like that term is mostly used for misheard phrases in songs and poems, not ordinary conversation. – Barmar Apr 22 '14 at 12:22

According to Merriam-Webster, "inside out" implies that something is turned inside out, like an article of clothing. This is a literal sense. Additional definitions are more figurative, "knowing someone inside out" is to know them thoroughly.

"inside and out" is in Merriam Websters abridged dictionary, and is therefore not available online. It does cite "inside out."

"Inside and out" can mean simply the inside and the outside. "Inside out" implies a turning inside out, to put it extremely, a disemboweling, decomposing, or undoing.

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As a native British speaker of English, I can only attest to knowing something "inside out." The alternative variation does sound rather strange but that, I'm afraid, is a personal view.

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Interestingly enough, to my American ears, saying 'I know it inside out' (omitting 'and') sounds strange. It comes across as 'I know it the reverse of the way one normally knows it.' Also a personal view, for what it's worth. – cobaltduck Apr 22 '14 at 15:27

American (Midwest) here: I've always said and heard "inside and out"--except when it's been "backward/s and forward/s" (or the latter's reverse). It's something about the rhythm.

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