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As far as I can tell, they both mean something like stretch or extend. When
would you use each one?
CC BY-SA 3.0
2013-07-20T20:05:53.263
12380
2013-07-24T15:20:57.783
3213
question
words nuances word-choice
Difference between 延ばす and 伸ばす
Try converting のばす into Kanji and your IME should have a pop up that tells you
the difference.

Mine (Google IME) says:  

> **伸ばす**  
>  1.長くする。「爪を伸ばす」「路線を伸ばす」  
>  2.まっすぐにする。「背筋を伸ばす」「しわを伸ばす」  
>  3.勢いなどを増やす。「勢力を増やす」  
>
>
> **延ばす**  
>  1.時間を遅らせる。延期する。「締め切りを延ばす」「出発を延ばす」  
>  2.〈「伸ばす」とも〉既存のものに付け加えて長くする。「路線を延ばす」

So...  

> **伸ばす** for  
>  1\. Making something longer. Ex. growing your nails, extending a route.  
>  2\. Making something straight. Ex. straightening your back or smoothing a
> wrinkle.  
>  3\. Increasing things like power/momentum. Ex. extending one's influence or
> power.  
>
>
> **延ばす** for  
>  1\. Making the time of something later. Postponing something. Ex. extending
> a deadline or departure time.  
>  2\. Adding to something already existing such as a route to make it longer
> (`伸ばす` also OK for this).
CC BY-SA 3.0
2013-07-20T20:54:34.570
12381
2013-07-20T21:01:16.877
2013-07-20T21:01:16.877
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12380
answer
延ばす is mainly used for extending "time or duration"

伸ばす is mainly used for extending "something tangible"
CC BY-SA 3.0
2013-07-24T15:20:57.783
12416
2013-07-24T15:20:57.783
3755
12380
answer