From an interaction design perspective, it's useful to look at augmented and virtual reality not as two distinct technologies, but rather as points on a spectrum that covers various combinations of simulated realities.¹ Augmented reality might typically overlay visuals on the "real" world, and virtual reality typically denotes a all-encompassing virtual world isolated from the real, but there is room in the design space to move between these two ends of the spectrum.

VRtuos is an excellent example of the "somewhere-in-between". It is a way to learn to play the piano with a VR headset — Guitar Hero or Tap Tap Revenge style. Users pair VRtuos to their piano and load a MIDI file. Now the notes to the song come falling down from virtual skies, yet physically mapped to the piano keys. It's really a clever way to combine digital content with physical input. This particular "point" on the AR/VR spectrum has been called Synchronized Reality² by a previous professor of mine, and seems like a promising way to interact with AR/VR worlds — building on familiar interaction possibilities and strengthened by physical feedback, yet offering totally novel functionality.