When describing bends in piping, joints, and in many other contexts the word "elbow" is used. You go to the hardware store and you see 90 degree elbows. The word elbow is used everywhere. Why not knee? Why elbow?

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    I really doubt you will get any answer better than "because".
    – terdon
    Commented Sep 4, 2013 at 16:19
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    I'm not sure anyone can say why something didn't happen.
    – Andrew Leach
    Commented Sep 4, 2013 at 16:21
  • Then they could say why we accepted elbow to mean a bend instead of knee. Commented Sep 4, 2013 at 16:23

3 Answers 3


Well take a look at this picture (knee brace anchor):

enter image description here


  • I wonder if there are certain bends where knee is more accepted. Commented Sep 4, 2013 at 16:24
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    Maybe those which tend to be in a lower position, as opposite to something which is in a upper position. Just guessing: know nothing about plumbing. :-))
    – Pam
    Commented Sep 4, 2013 at 16:34
  • Not a master plumber but an elbow piece can be used high or low. I don't see the word knee much so I don't know about that. Commented Sep 4, 2013 at 16:40
  • @RyeBread: A Knee is typically a bend that connects it to a stabilizer or support.
    – Jacobm001
    Commented Sep 4, 2013 at 18:46
  • @Jacobm001 - that might be an answer. Commented Sep 10, 2013 at 18:33

There may be a very simple explanation for the "elbow" phenomenon. When was the last time you used your leg to illustrate nonverbally a point you were making--other than while demonstrating a style of kicking?

People, by and large, tend to use hands and arms to gesture. They do so to illustrate, to emphasize, and to show approbation or disapprobation (e.g., the A-OK sign, as with the index finger forming an "O" with the thumb; or with the thumbs-up sign; or the not-so-OK sign, as with "thumbs down" or with the middle-finger salute!). With legs, not so much.

As Pam points out, the knee simile is used, it's just not as commonly used and understood as well as the elbow simile.

  • Not to mention that most likely when the terms were adopted, women would have been unable to use "knee" for illustrative purposes without causing scandal.
    – JeffSahol
    Commented Sep 4, 2013 at 18:13
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    @JeffSahol: True enough! By the way, do plumbers still talk about female- and male threads, as in pipe threads? If so, how un-PC of them! Better, I suppose, would be "inside threads" and "outside threads." I know they still use the term "nipples," but then that is a unisex sort of word and not un-PC. Commented Sep 4, 2013 at 18:38

While I can't find a source that really defines it, it's been my experience that an elbow is traditionally a twist or bend in a pipe. A knee on the other hand, is typically a bend that connects to a joist or other kind of support structure.

In Pam's example it's a pivoting mount that connects to a support post.

  • I think this is it. Will add a check if someone can give more examples. Commented Sep 11, 2013 at 16:28

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