Crystals is a microtonal, computer-assisted composition scored for 16 strings. It lasts approximately 17 min and is the first in a series of explorations of large-scale, intrinsic forms scored for live performers. (Other works in this series, all computerassisted, include Hockets for 9 trombones, 1980 [approximately 35 min]; Protocol for solo piano, 1981 [approximately 17 min]; and a work in progress.) I have long held the conviction that music should be able to stand on its own merits as sound, and in recent years I have also formed the opinion that the critical test of a compositional approach is its ability to sustain a work esthetically over an extended duration. This attitude has caused me to focus much attention on the linkage between the structure of a composition as it exists on paper and the way listeners perceive that structure. If listeners cannot hear the structure or at least learn to hear it, it might as well not be there. My first inspirations for Crystals date back to the spring of 1979. James Tenney traveled to Buffalo from Toronto several times to lecture on a variety of people and topics: Charles Ives, Harry Partch, his (Tenney's) experiences at Bell Laboratories, his current interests, and his model of how we perceive musical structures. My own concerns made me especially receptive to this last topic. Tenney had derived his model from Gestalt psychology, particularly its fundamental assertion that we perceive structure only in relation to context. Gestalt psychology was first advocated during the 1920s by such figures as M. Wertheimer, K. Koffka, and W. K6hler. It was Wertheimer (1938) who demonstrated two transcendent factors in the formation of perceptual aggregates: (1) that of relative proximity in one or more perceptual dimensions and (2) that of relative similarity in one or more aspects of shape. Tenney applied these factors to the dimensions and shapes of music. He pointed out how proximities in dimensions such as time and register and similarities in aspects of shape such as rhythm and intervallic contour link not only local details but even global forms into larger structures. Conversely, distance in these dimensions and contrast in these aspects of shape work to isolate the various components of a musical aggregate. When we listen to a musical work, the most dramatic changes provide our landmarks-an emphatic cadence upon a new tonic in tonal music, a great shift in texture or color in all music-these are the cues we depend on to discern when one section of a form gives way to another. The forces of relative cohesion and segregation thus establish a perceptual hierarchy of which the total experience of the composition forms the apex. Moving down this hierarchy, the composition divides and redivides into progressively smaller units until the most elementary sounds, typically the individual notes, are isolated. Tenney provided important models for Crystals in a series of early works including Phases, written in 1963, one of the first compositions both and realized using the digital computers at Bell Laboratories, and Music for Player Piano, composed in 1964 (Tenney 1969). (What sets Crystals apart from these works is more a matter of method than of concept. Tenney's implementation was not recursive, although his model clearly is. I have replaced his system of averages with the various systems of nested constraints described in this article, allowing more coordination among simultaneous aggregates and greater isolation among distantly related ones. I have also exploited harmony as an independent perceptual domain; harmonic relationships in Phases and Music for Player Piano arose only coincidentally from Tenney's registral proceCopyright C 1982 Charles Ames. All musical examples are also copyrighted by Charles Ames.