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It seems they can be used interchangeably but I am not sure. Can somebody explain the difference of use between these two key words? Where do they become different?

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  • You certainly can't use them interchangeably in all contexts - in can be used in many places where within is totally wrong. If OP is unsure, I suggest sticking to in, which would rarely if ever be unacceptable. Aug 31, 2011 at 17:32
  • 2
    Many of my high school students (I am a History teacher) like to use "within" in places where "in" would do, because they think "within" sounds more scholarly. So I get essays with titles like "Authoritarianism Within Putin's Russia" or "Communist Economic Policies Within Cuba." The use of "within" in these titles makes it sound as if authoritarianism and communist policies are physical things that somehow float around inside the borders of those respective countries. So I encourage students to use "in" whenever it makes sense. (But it's difficult to convince a young, ambitious student to use
    – Alex
    Jun 9, 2019 at 13:44
  • ...a small word in place of a longer word!) @Alex
    – Mari-Lou A
    Jun 9, 2019 at 14:41

3 Answers 3

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Within is closer to inside - it mainly refers to something like a building where you are physically in (or inside).

In can also mean a state or an organisation, so you can be "in trouble", or "in school" but not "within trouble".

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  • The other answer makes a good point too: You can't always substitute in for within (or inside). Apr 20, 2013 at 11:38
  • @mgb What about the example sentence "the algorithm is able to detect people in/within google-earth images"? Is within acceptable/preferable in this case?
    – halirutan
    Aug 12, 2016 at 10:17
  • @halirutan, no 'within an image' wouldn't be normal, Generally always use 'in' unless you know different.
    – mgb
    Aug 12, 2016 at 19:11
  • @mgb, For <within> vs <inside>?
    – Pacerier
    May 9, 2017 at 17:39
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In- A limit of some sort.

i.e. "In 6 days,..."

You limit the time in the sentence exactly.

Within- Inside the said limit.

i.e. "Within 6 days,..."

You limit the time in the sentence, but inside the said time.

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where ever there is a certainty we use in and in uncertainty we use within.

I will come in next week. - Use IN-
I will come within this week - Use within

Also, if some timeframe is allocated ie : do the work in ten minutes

Then if the work is completed in 10 min then use in and if completed before 10 min then within.

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  • 1
    I will come in next week does not mean (what I surmise you are trying for) "I will come in the next week". "I will come within the week" means the latter, but the way to express your last point is "I will come in a week's time". Apr 20, 2013 at 11:43

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