Hosa GPP-419 is an adaptor designed to adapt a mono phone plug to a stereo phone input. It applies mono equally to stereo left and right.
Mono 1/4" Female to Stereo 1/4" Male
Female end accepts a mono signal and applies it to both Tip and Ring contacts at the male end. Use it to send a mono signal to both Left and Right contacts in a stereo 1/4" jack. Also can be used to "trick" and audio mixer's insert (sent / return) jack into behaving like a direct output.