This dissertation analyzes the forms of the preludes in J.S. Bach’s Well Tempered Clavier. The prelude originated as an improvised genre in the mid-fifteenth century. Treatises of the period gave instruction on how to improvise and eventually compose preludes and that tradition informed Bach’s compositional practice. However, an additional genre, the invention, played a similarly important role in influencing the composition of the preludes in the Well Tempered Clavier, particularly in Book II. I analyze the forms of the preludes by proposing a new method: dimensional interplay. This method examines the form of each prelude in three musical dimensions: melody, tonal structure, and texture. Moments of change within each dimension create junctures that separate sections of music. The dimensions often change at different times, thus creating three different potential forms for each prelude. I combine these three forms to discover places where junctures in various dimensions align to create strong indications of a change of section. I then use the resulting forms to sort the preludes into types and subtypes. Most preludes fall into two broad categories: textural preludes (similar in many ways to the pattern prelude proposed by Jon Lester in 1998 and 1999) and melodic preludes (similar in many ways to Bach’s inventions). Despite the prevailing notion that the prelude is a highly variable and open-ended form, this dissertation shows that Bach’s WTC preludes operate within a formal framework.