The sonata form is a guide to composers as to the schematic for their works, for interpreters to understand the grammar and meaning of a work, and for listeners to understand the significance of musical events. A host of musical details are determined by the harmonic meaning of a particular note, chord or phrase. The sonata form, because it describes the shape and hierarchy of a movement, tells performers what to emphasize, and how to shape phrases of music. Its theory begins with the description, in the 18th century, of schematics for works, and was codified in the early 19th century. This codified form is still used in the pedagogy of the sonata form.