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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In the fist sentence **Quite** refers to **Completely**. While in the second sentence **Pretty** refers to **a certain extent**.