American organ music of the second half of the nineteenth century is neither well known nor widely played, often relegated to performances on historic American organs rather than occupying an established place in the canon. Similarly, scholarship on this music is lacking. While there is abundant material treating late nineteenth-century organs, there is little writing about the music and its performance. Among the performance topics least treated in this scholarship is registration. The inherent variability of registration as a practice, adapting registration suggestions as needed depending on the organ, is a barrier to its study, and previous examinations of registrations are too vague to communicate applicable concepts. Many treatises and tutors about organ playing were published in the second half of the nineteenth century. These documents offer insights into both the actual registrations used as well as the theory behind their selection. Writers used analogies to painting and orchestration to convey their views of registration. The motivation behind these comparisons was to describe the process of registration adaptation based both on practical necessity and personal taste. The development of the orchestra, orchestral instruments, and orchestral writing allow the analogies of writers on registration to be interpreted through the current practice of their time. This reveals an understanding of registration that is flexible and relies more on the organist’s opinions and tastes than the printed indications might suggest. One common practice of the second half of the nineteenth century was to indicate registration solely through the use of dynamic markings. Treatises and tutors of the time commonly included registration tables describing the creation of a crescendo with stops. These tables list various combinations of stops from pianissimo through fortissimo, revealing a spectrum of sounds that can be used to interpret the dynamic markings in the published music. Dynamic markings are thus revealed to be communicators not only of volume but of timbre, communicated through the suggested combinations of stops. Examining the treatises and tutors in addition to the published music that does contain specific registration indications reveals the ways in which stops were used when a piece calls for a more characteristic or solo/accompaniment registration than the general registration tables would convey. This serves as an interpretive key to pieces that contain no registration markings at all as well as serving as a basis for adjustment and alteration. Of central importance to this dissertation are George Elbridge Whiting’s Organ Accompaniment and Extempore Playing (1887), Dudley Buck’s Illustrations in Choir Accompaniment (1892), and Everett Truette’s Organ Registration (1919). Unlike tutors, which contained only brief instructions at the front and were mostly musical examples intended for completely inexperienced organists, these three major treatises are intended for students with advanced levels of ability on the instrument. Coupled with the study described above, the extensive writings of Whiting, Buck, and Truette reveal a registration practice that went beyond the markings on the page, relying on them for initial guidance but then both allowing and expecting the organist to orchestrate the piece in accordance with their taste and the character of the music.