"Fat alley cats eat garbage" is a mnemonic device used for learning how to play music on the C clef centered to the third line of the staff, also called the alto clef, which in modern musical performance is most frequently used by performers on viola and mandola. The words of the phrase each represent the pitch, from lowest to highest, of one of the lines on the musical staff, with C being the middle (third) line, F being the lowest line, and G being the highest line.
The mnemonic device for the spaces on the staff in alto clef is "Good boys deserve fudge."
The best-known equivalent mnemonic devices for treble clef (G clef centered to the third line) are "Every good boy does fine" for lines, and the word "Face" for spaces.
The bass clef (F clef centered to the fourth line) uses the mnemonics "Good boys do fine always" for lines (which understandably gets confused with the treble mnemonic, among child learners) or the alternative "Grandpa Bach did fugues aplenty," and "All cows eat grass" for spaces.
Another C clef, the tenor clef, centers on the fourth line, and mnemonics I have heard for its lines are "Dogs face a cat eagerly," and for its spaces, "Eat green bananas daily."
Iron Noder 2021, 25/30