Final answer:
A Perfect Fifth below G-flat (Gb) is C-flat (Cb). To determine this, count seven half-steps down from Gb, which leads you to Cb. The mnemonic 'Every good boy does fine' is a mnemonic device used in music theory.
Step-by-step explanation:
The question asks what is a Perfect Fifth below G-flat. A Perfect Fifth interval consists of seven half-steps. To find a Perfect Fifth below G-flat (or G♭), you need to count seven half-steps down from G♭. Starting from G♭, the notes you pass, moving down in half-steps, are F, E, E♭, D, D♭, C, and finally, C♭. Therefore, a Perfect Fifth below G♭ is C♭.
The mnemonic "Every good boy does fine" is an example of a memory aid specifically known as a mnemonic device, which is used to help remember the order of notes on the lines of the treble clef in music notation.