In the song “Stink-foot” on the album Apostrophe ( ’ ) (1974), Frank Zappa wrote:
 
“The crux of the biscuit/ Is the Apostrophe.
 
“The crux of the biscuit” means the same thing as “the heart of the matter.” It’s not known why “biscuit” is used—perhaps because “crux” sounds like “crust(s).” The phrase “the crust of the biscuit” has long since dropped the “is the apostrophe” answer and has been used without reference to the Frank Zappa song.
 
   
Wikipedia: Apostrophe ( ’ )
Apostrophe ( ’ ) is an album by Frank Zappa, his eighteenth, released on March 22, 1974 in both stereo and quadraphonic formats. An edited version of its lead-off track, “Don’t Eat the Yellow Snow”, was Zappa’s first chart single, reaching position 86. Apostrophe ( ’ ) remains Zappa’s biggest commercial success in the US. It was certified Gold by the RIAA on April 7, 1976 and reached number 10 on the Billboard Top 200 Albums Chart.
 
Wikipedia: Frank Zappa
Frank Vincent Zappa (December 21, 1940 – December 4, 1993) was an American composer, singer-songwriter, electric guitarist, record producer and film director. In a career spanning more than 30 years, Zappa wrote rock, jazz, orchestral and musique concrète works. He also directed feature-length films and music videos, and designed album covers. Zappa produced almost all of the more than 60 albums he released with the band The Mothers of Invention and as a solo artist.
 
Free Merriam-Webster Dictionary
crux noun \ˈkrəks, ˈkru̇ks\
plural crux·es also cru·ces
Definition of CRUX
1: a puzzling or difficult problem : an unsolved question
2: an essential point requiring resolution or resolving an outcome “the crux of the problem”
3: a main or central feature (as of an argument)
Origin of CRUX
Latin cruc-, crux cross, torture
First Known Use: 1718
 
Google Books
20 March 1984, Boston (MA) Phoenix, “Candid camera” by Clea Simon, pg. 8, col. 3:
A complex work-in-progress produced by the band was shown in February at the Mobius Theater, an experimental outlet in Boston. Entitled The Crux of the Biscuit, this 15-minute tape opens with a shot of a woman in a barren room.
     
30 May 1990, Trenton (NJ) Evening Times, “Dykstra: Firing no surprise” by George A King III, pg. C5, col. 4:
The crux of the biscuit was Dykstra didn’t dig Johnson’s platoon system that had Dykstra sharing the center field job with Mookie Wilson.
 
6 December 1991, The Slate (Columbia, SC), “Blightobody plans to keep Blightogrowing,” pg. 6D:
Aha, the crux of the biscuit!
     
Google Books
Secrets from the Masters:
Conversations with forty great guitar players
By Don Menn
San Francisco, CA: GPI Books
1992
Pg. 267 (Steve Vai):
I don’t really care, because the crux of the biscuit is if somebody hears it and likes it, then that’s all that matters.
 
Google Books
The Words and Music of Frank Zappa
By Kelly Fisher Lowe
Westport, CT: Praeger
2006
Pg. 113:
Although it does tease the audience with the statement, “The crux of the biscuit/ Is the Apostrophe ( ’ ).” it does not, I feel go much beyond that.
 
OCLC WorldCat record
“The Crux of the Biscuit ...” : Über politische und andere “Atrocities” in Frank Zappas Musik
Author: Thomas Phleps
Publisher: Giessen : Giessener Elektronische Bibliothek, 2008.
Edition/Format:  Computer file : German
 
Yahoo! Answers
What does this mean? The crux of the biscuit is the apostrophe?
4 years ago
pinecone…
Best Answer - Chosen by Voters
it’s from a frank zappa song. the crux of the biscuit means the heart of the matter. he had an album called “apostrophe” so maybe it was an homage to his own album!! he had the whole line in a song on the album. i am not a fan of his, i just looked it up because i loved the question. and it was also suggested somewhere that crux of the biscuit is a combined phrase from the crust of the biscuit and crux of the matter…meaning that the crust of the biscuit is all someone left you-nothing, etc but crux of the matter is the important part. so there you go.
   
Dweezil Zappa World
The Crux of the Biscuit is the Apostrophe
by Kurt - Son of the ROXY wrote on Mar 8, 2010 at 05:47 AM
Kurt - Son of the ROXY’s Blog Posts
...the particular passage which is being referred to happens on the the end of ‘stinkfoot’, and takes place in the context of a man having a conversation with his dog.
the dog asks the man, what is his conceptual continuity.
so the dog is posing a philosophical question to the man about what the man thinks the nature of reality is, or at least his understanding of it.
the dog is basically asking, what is reality?
well, what is an apostrophe?
an apostrophe is a symbol. its an idea. and what is anything else for that matter? what am i to you, but an idea within your mind.
and so, the apostrophe, being a symbol of defined purpose but undefined potential, becomes a more accurate symbol to represent whatever we currently call ‘reality’.
basically saying that the crux of the ‘biscuit’, i.e. reality itself, is more accurately represented by a symbol such as an ‘apostrophe’.
meaning… words are limited, language is limited…
   
Google Books
A Corpse’s Nightmare
By Phillip DePoy
New York, NY: St. Martin’s Minotaur
2011
Pg. 244:
“He is, apparently, the crux of the biscuit.”
“I hate that phrase,” Andrews mumbled.