 GM2 maintains backward compatibility with GM, but increases polyphony to 32 voices, standardizes several controller numbers such as for sostenuto and soft pedal ("una corda"), RPNs and Universal System Exclusive Messages, and incorporates the MIDI Tuning Standard.GM2 is the basis of the instrument selection mechanism in Scalable Polyphony MIDI (SP-MIDI), a MIDI variant for low power devices that allows the device's polyphony to scale according to its processing power.Most MIDI synthesizers use equal temperament tuning.