In this paper, we discuss a design principle for the musicalinstruments that are useful for both novices and professionalmusicians and that facilitate musically rich expression. Webelieve that the versatility of conventional musicalinstruments causes difficulty in performance. By dynamicallyspecializing a musical instrument for performing a specific(genre of) piece, the musical instrument could become moreuseful for performing the piece and facilitates expressiveperformance. Based on this idea, we developed two new typesof musical instruments, i.e., a "given-melody-based musicalinstrument" and a "harmonic-function-based musicalinstrument." From the experimental results using twoprototypes, we demonstrate the efficiency of the designprinciple