Paper-in-Oil Tone capacitors for Guitar and Bass

-  PIO stands for Paper-In-Oil, and it refers to a specific type of capacitor construction used in the passive tone
    control circuits of electric guitars, especially in models aiming for a vintage. This construction is similar to the
    capacitors used in many vintage guitars (like the famous "Bumblebee" caps in 1950s Gibson Les Pauls) and is 
    what gives them their unique reputation.