Vala or The Four Zoas is one of William Blake's few surviving manuscripts and affords a unique opportunity to examine a significant evolution in his poetic practice. While the poem itself exhibits a consistent thematic interest, the modes and methods of representing these interests underwent a radical change in the ten or more years in which Blake wrote and reworked the poem. Flexible Design offers an extended and detailed treatment of the gradual shift that took place in Blake's poetics during the composition, transcription, and revision of Vala or The Four Zoas.

Using the idea of a "flexible design," John Pierce examines the ways in which Blake's mythology and his poem possess a flexibility that allows for significant change to characters, symbols, and poetic techniques within a previously constructed framework. Pierce traces how, in the process of revision, Blake experimented with characterization, increased the importance of Christian symbolism, and developed a mode of narrative presentation controlled less by chronological sequence than by the use of thematic juxtaposition and typology.