  Learning a musical instrument requires a lot of practice, which ideally,
should be done every day. During practice sessions, students are on their own
in the overwhelming majority of the time, but access to experts that support
students "just-in-time" is limited. Therefore, students commonly do not receive
any feedback during their practice sessions. Adequate feedback, especially for
beginners, is highly important for three particular reasons: (1) preventing the
acquirement of wrong motions, (2) avoiding frustration due to a steep learning
curve, and (3) potential health problems that arise from straining muscles or
joints harmfully. In this paper, we envision the usage of head-mounted displays
as assistance modality to support musical instrument learning. We propose a
modular concept for several assistance modes to help students during their
practice sessions. Finally, we discuss hardware requirements and
implementations to realize the proposed concepts.
