Since the late nineteenth century, scholars have formulated theories about the role and placement of organs in the St. Matthew Passion, using them to interpret the work's theological message and anchor broader ideas about how Bach positioned his other players and singers for performances in the Thomaskirche. The church housed two permanently fixed organs: a large one in the west gallery and the misleadingly labeled swallow's nest organ in a gallery on the east wall. The latter, smaller organ has become the central point in debates about the Passion, leading to a variety of interpretations. The current evidence, reviewed here in substantial detail, does not unequivocally prove any of the main theories. Yet not all are equally strong, By examining the history of scholarship on the St. Matthew Passion and by carefully reexamining the evidence, we can grow more aware of weaknesses stemming from the enduring influence of older scholar ship, based on questionable premises, outdated information, or seemingly unintentional misreading. The results argue strongly against one of the most common theories, which posits a small auxiliary role for the east gallery organ, along with a "third choir," for most of Bach's performances. Proponents of this theory have sometimes harnessed it to add another layer of theological symbolism to Bach's music. This too is questionable. Musical evidence may also argue against it. Bach's changing approach to scoring the chorale "O Lamm Gottes, unschuldig in the opening movement shows that the melody does not work in dialectic opposition to the two main choruses, as sometimes claimed, but emerges from within the group. In place of this theory, two alternatives stand out: either the full separation of the two choirs beTween the two galleries of the Thomaskirche or the positioning of all performers in the west gallery, with no role for the east gallery or its organ for any of Bach's Passion performances. Though neither fully accounts for every detail in the evidence nor answers all possible objections, they do offer more satisfying explanations than the alternatives. [ABSTRACT FROM AUTHOR]