The Letter is a musical composition by the American composer Harry Partch (1901-1974). Although its title implies a singular creative entity, The Letter is in fact comprised of four distinct versions, composed between 1943 and 1972. I have also elected to consider the various revisions applied to the first version (Letter from Hobo Pablo) as a fifth version of the work. The Letter is a highly significant and unique work within the context of Partch's practice. Unlike any of his other compositions, its numerous distinct incarnations delineate a trajectory that encapsulates almost the entire chronology of his creative practice. Furthermore, they function as models for the composer's development of new instruments, new performance strategies, new technologies for documentation, and new compositional methodologies. Functioning in parallel as a collective of multiple versions, The Letter offers a recurring and developing snapshot of Partch's life on both a personal and artistic level. His recurring creative engagement with, and re-assessment of this particular work offers fundamental insights into the multi-faceted, integrated and yet always pragmatic nature of his creative processes and philosophical intentions. This thesis aims to understand what The Letter is: how its constituent versions differ from each other, as well as to investigate their complex inter-relationships. An exploration of the specific creative processes and the biographical contexts that informed them provides insights into Partch's shifting artistic ideals in relation to his primary focus for each of the versions of The Letter, and how these would eventually manifest - as a score, performance, recording, new instrument, or film. The submitted practical outcomes of this thesis aim to reflect the focus of Partch's labours for each version of The Letter, as well as reinforce the lack of any definitive version or performance representation of the work. The first chapter of this thesis explores not only the biographical contexts that provided the original textual impetus for the work (a letter by Partch's companion Pablo), but also contains an in-depth theoretical analysis that aims to demonstrate the structural, organological and material components that would be retained within all later versions of The Letter. As a creative response to the paucity of its performance history, the first practical outcome of this thesis is a filmed concert performance of the first version of the work (Letter from Hobo Pablo, 1943), played on replica instruments (e.g. the first Adapted Guitar 1 and Kithara 1). In Chapter Two, an investigation and detailed analysis of the various revisions applied to the first version of The Letter - as well as a consideration of this fifth version's organological implications on subsequent incarnations of the work - has resulted in the creation of a legible performance edition of Partch's original revised score. This document also includes a transcription of his instruments' various idiosyncratic tablatures into standard equal temperament notation. The third chapter considers the biographical, technical and organological contexts that informed the composer's (eventual) primary focus of the second 'official' version of the work (The Letter, 1950) - recording and the recorded medium. Previously unheard recordings of proto-versions of a number of pieces (including The Letter), recorded shortly before this latter work's second version was composed, are discussed, as well as Partch's application of recorded objects (vinyl records) within his own performance practice. The chapter's practical outcome, a filmed recording of a hybrid version of the second and third versions of The Letter, aims to portray the nature of the recording process and studio environment, as well as the particular visual architecture of Partch's layered scores. The focus of the third version of The Letter (1955) - new instruments - is explored in Chapter Four through an investigation of the origins of Partch's corporeal principles and their earliest applications in his compositions. An intense period of revision to a number of his works in the mid-1950's - as well as the concurrent development of a number of new instrumental designs - shifted the outcome of the third version of The Letter such that its eventual recording did not satisfy the composer's creative ideals. This chapter's practical outcome was the building of a replica of one of those instruments first incorporated into the third version of the work - the Surrogate Kithara. Additionally, the research required to replicate this instrument coincided with the discovery of a forgotten prototype - the "Oakland Double Canon". Chapter Five outlines the biographical contexts that influenced the creation of the final version of The Letter (1972): the collaborative relationship between the composer and Stephen Pouliot, director of the documentary The Dreamer That Remains, and the new instruments and recording strategies that were developed in order to solve the creative challenges of realising this, the most complex amalgam of Partch's creative focus in relation to The Letter. This chapter's performative outcome aims to embody the individualised performance representation of Partch's practice within the documentary, by means of a recording of Letter from Hobo Pablo, arranged for a solo performer (myself) utilising voice and his replica of Partch's first Adapted Guitar 1. The presentation of a diverse set of practical outcomes within this thesis aims to reflect Partch's rejection of artistic specialisation, as well as reinforce the non-hierarchical nature of the various incarnations of The Letter, where no single version of the work functions as a singular, definitive creative statement.