A complement to the registered parameter mechanism of the MIDI Controller message type. The non-registered parameter numbers, or NRPNs, have no conventional meanings assigned to them by the MIDI standard; they are available to each manufacturer to assign meanings to as they wish.
A non-registered parameter is selected for editing by sending the controller message numbers 98 and 99 to the target synth, containing the parameter number; controller 98 is the high-order byte of the number, and controller 99 is the low-order byte. The procedure for actually changing the value is the same as for a registered parameter; see that entry.