In this paper, we discuss a design principle for the musical instruments that are useful for both novices and professional musicians and that facilitate musically rich expression. We believe that the versatility of conventional musical instruments causes difficulty in performance. By dynamically specializing a musical instrument for performing a specific (genre of) piece, the musical instrument could become more useful for performing the piece and facilitates expressive performance. Based on this idea, we developed two new types of musical instruments, i.e., a "given-melody-based musical instrument" and a "harmonic-function-based musical instrument". From the experimental results using two prototypes, we demonstrate the efficiency of the design principle.