Set a drone humming and a bodhrán going, then play your tune over the top. Every sound is made in the browser as it plays, with no recordings.
Open is root and fifth only, which sits under any mode. Major adds a soft major third (suits major and Mixolydian tunes); Minor a minor third (Dorian and Aeolian).
Steady keeps to the beat. Session plays the groove with a variation every fourth bar and a fill every eighth. Busy varies more often.
Space bar starts and stops. Changes take effect as it plays: the drone glides across, the drum switches at the next bar line.