This thesis is an investigation of the formal and phenomenological characteristics of a specific spatial morphology: narrow, tall, long, habitable space which occurs between at least two other spaces whether connected or not. This investigation does not aim to address all types of interstitial space. The general form of the space is the constant; variables include size, scale, modulation, intended and unintended uses. The method of investigation will be the creation of a matrix of examples that allows sorting by the various categories. It is expected that examining sorted categories will lead to a comprehensive and profound understanding of this spatial type. And that this, in turn, will prompt a program that will provide the basis for a demonstration of the range of uses of this spatial type. An investigation of the interstitial is intended to illuminate the author’s affinity and intuitive use of this spatial form in her work and to generate a framework for the author from which to approach architecture, using interstitial space intentionally as the primary organization and driver of form in future explorations.