Question: What is the difference between *quite* and *pretty* in the following context:

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> The differences between these concepts are quite complicated.
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and

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> The differences between these concepts are pretty complicated.
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Answer: 
Firstly, when you search the word 'quite' on OALD, it says *pretty* is a synonym.

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> [quite (synonyms - fairly, pretty)](http://www.oxfordlearnersdictionaries.com/definition/english/quite) - *to some degree*
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But then, if you search for *pretty*, besides its general meaning, it also means *very* *[I actually thought it this way when I saw the sentence first!]*

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> [pretty](http://www.oxfordlearnersdictionaries.com/definition/english/pretty_1) - *very*
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So...

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> The differences between these concepts are quite complicated = *to some degree complicated.*
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> The differences between these concepts are pretty complicated = *to some degree complected* **OR** *very complected*
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*Quite* means, variably:

* *Exactly* or *completely*, as in "quite so" (meaning "exactly right")
* *Somewhat* or *fairly*, as in "quite big" (something cannot be "completely big", so this must mean "fairly large")

*Pretty* means the same as that second sense of *quite* (In this context! Obviously it also means cute/beautiful etc.), so that one can say "pretty big" but not "pretty so".

Unfortunately, there is a tendency to use BOTH of these words ironically or sarcastically, for example:

* Describing something extremely large as "pretty big" or "quite big" (the second sense of *quite*)
* I'm quite sure there must be a pretty good example of the first sense of *quite*, but I can't think of one :(

To answer the question, there is no real difference between the two words in the context of the question - *quite* doesn't necessarily indicate the degree to which something is [adjective], any more than *pretty* does.

It's also worth noting that the second sense of *quite* is more common as a colloquialism in British English than *pretty*, whereas the first sense you would be more likely to see in polite/formal written language.