Seixas' keyboard works were written for a variety of instruments, including the organ, harpsichord, and the clavichord. Stylistically speaking, however, his sonatas showcase a range of musical styles: some are exemplary of a Baroque toccata; some are firmly in the galant style; some are clearly influenced by the German "Empfindsamer Stil" (literally 'sensitive style'). Despite rarely, if ever, traveling outside of Lisbon, his work also includes various geographical styles, such as the German Mannheim school, the French minuet, and the Italian style as composed by Scarlatti, his colleague and contemporary. Santiago Kastner, Seixas' biographer and editor of his pieces, describes Seixas' works as "unoccupied" with a particular form, and given over to frequent improvisation. Much of his work was destroyed in the earthquake that devastated Lisbon in 1755. Only three orchestral pieces and around one hundred keyboard sonatas out of over an alleged seven hundred survived, plus a handful of choral works for liturgical use (much more conservative than what one would expect from his instrumental music).