With its patented, vertical touch detection, the Verti-Chord Keyboard app provides 3 octaves of full-sized piano keys on-screen at any given time, thus making it the ONLY piano app suitable for real-time playing.
UPDATE: Sorry to say, this app is no longer compatible with the latest iOS. If you are running iOS 10+, you should upgrade to the Verti-Chord Pro app (also FREE).
If, however, you wish to continue using this app, be sure to make any desired instrument/voice changes immediately upon restarting the app, before any notes have been sounded; otherwise, such changes will cause the app to crash.