“Why are orchestra intermissions limited to twenty minutes?” (joke)

A classic joke is said about orchestra intermissions:
 
Q: Why are orchestra intermissions only twenty minutes long?
A: So the violists don’t need to be retrained.
 
The joke has been cited in print since at least 1991 and has been said about violists, cellists and drummers.
     
 
Google Books
November/December 1991, Gasbag (Friends of the University of Michigan Gilbert And Sullivan Society), “Viola III” administered by David Goldberg, pg. 19, col. 2:
Q: Why are orchestra intermissions only twenty minutes long?
A: So the violists don’t need to be retrained.
   
Google Groups: rec.music.classical
finally - the joke list. . .
susan poliniak
11/10/93
(...)
Q - Why are orchestra intermissions only twenty minutes long?
A - So the violists don’t need to be retrained.
   
Google Groups: rec.humor
Canonical List Of Music Humor 1/1
Too Live Krewe
2/15/95
(...)
Why are orchestra intermissions limited to 20 minutes?
So you don’t have to retrain the cellists.
 
Google Books
The Musician’s Joke Book:
Knowing the Score
By Nancy Groce
New York, NY: Schirmer Books; London: Prentice Hall International
1996
Pg. ?:
Why are orchestra intermissions limited to 20 minutes? So they don’t have to retrain the cellists.
 
Google Books
The Best Musicians’ Jokes
By Bruno Kassel and Carlo May
Pacific, MO: Mel Bay Publications, Inc.
2005
Pg. 39:
Why are orchestra intermissions limited to 20 minutes?
So they don’t have to retrain the drummers.
 
Google Books
The Big Ass Book of Jokes
By Rudy A. Swale
Berkeley, CA: Ulysses Press
2009
Pg. 374:
Q: Why are orchestra intermissions limited to 20 minutes?
A: So you don’t have to retrain the cellists.
 
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Canadian Musician Magazine
Joke of the Day
Why are orchestra intermissions limited to 20 minutes?
So you don’t have to re-train the cellists.
November 17, 2014