As a response to a number of notable contemporary aesthetic tendencies, this paper introduces the notion of an infrainstrument as a kind of ‘new interface for musical expression’ worthy of study and systematic design. In contrast to hyper-, meta- and virtual instruments, we propose infra-instruments as devices of restricted interactive potential, with little sensor enhancement, which engender simple musics with scarce opportunity for conventional virtuosity. After presenting numerous examples from our work, we argue that it is precisely such interactionally and sonically challenged designs that leave requisite space for computer-generated augmentations in hybrid, multi-device performance settings. Keywords Infra-instruments, hyperinstruments, meta-instruments, virtual instruments, design concepts and principles.