In this paper, a function-oriented musical interface, named the sound wheel_x0011_,is presented. This interface is designed to manipulate musical functions likepitch class sets, tonal centers and scale degrees, rather than the _x0010_musicalsurface_x0011_, i.e. the individual notes with concrete note heights. The sound wheelhas an interface summarizing harmony theory, and the playing actions haveexplicit correspondencewith musical functions. Easy usability is realized by semi-automatizing theconversion process from musical functions into the musical surface. Thus, theplayer can use this interface with concentration on the harmonic structure,without having his attention caught by manipulating the musical surface.Subjective evaluation indicated the e_x001B_ffectiveness of this interface as a toolhelpful for understanding the music theory. Because of such features, thisinterface can be used for education and interactive training of tonal musictheory.