Two instrument to one amp selector, imagine playing bass and on gig’s you’re using two basses to the same amp, the problem thought is they have not the same output, and you then have to first bypass the amp’s to shift instrument them adjust gain to have the same level on both instruments.

The solution to the problem is the Match Box, with it’s select switch that simply route either of the instrument through a gain circuitry to the amp, so you not only can shift between the two instruments, but also adjust the gain, so you don’t have to bypass the amp and adjust gain every time, you want to change instrument. Needless to say the Match Box of cause works just as well for guitar.