Final answer:
A Perfect Fifth below D-flat is G-flat, reached by counting down 7 half-steps from D-flat. Additionally, "Every good boy does fine" is a mnemonic device used to remember the lines of the treble clef.
Step-by-step explanation:
When you are determining intervals on the piano, knowing that a Perfect Fifth below a certain note means counting down 7 half-steps (or semitones) from that note. In this case, to find a Perfect Fifth below D-flat, you would count down 7 half-steps starting from D-flat. This would lead you to G-flat as the Perfect Fifth below D-flat, as the notes you would count down are: D-flat, C, B, B-flat, A, A-flat, and finally G-flat.
The mentioned mnemonic "Every good boy does fine" is a tool used to remember the notes on the lines of the treble clef (E, G, B, D, F), and it is an example of a mnemonic device, a learning technique that aids information retention.