Carlos de Seixas was the most prominent Portuguese composer and keyboard player of his day. He mainly wrote for the keyboard, particularly toccatas (sonatas), most of which have been lost. There are differing accounts about the number Seixas produced, but it appears he composed over 700 sonatas in his short lifetime, as reported in the Biblioteca Lusitana by Portuguese historian Ignacio Barbosa-Machado. In any event, fewer than 100 survive and only a mere handful of choral compositions are known, though it is almost certain he wrote numerous works in the sacred choral genre. There are various theories that attempt to explain the complete absence of Seixas' original manuscripts, the most prominent being that all were lost in the earthquake that devastated Lisbon in 1755. Only copies of the composer's scores survive, probably representing only a fraction of his output. At some point, probably in his late teens, Seixas discarded the family name of Vaz and used his first three names, José António Carlos, as several manuscripts attest. For unknown reasons, he adopted the surname (de) Seixas. He divulged talent early on, probably having been given his first music lessons by his father, the Coimbra Cathedral organist Francisco Vaz. In 1718, his father having died, Seixas was appointed organist at the cathedral. While it may involve a small measure of conjecture to conclude his talent must have been remarkable for him to attain such a prestigious position at the age of 14, it is completely safe to say that by age 16, he was one of the finest keyboard players in Portugal since he was appointed organist at the Royal Chapel in 1720. Later, while retaining the organist post, he became vice chapelmaster under chapelmaster Domenico Scarlatti. Some have suggested that the youthful Seixas was actually the more talented keyboard player of the two, citing a questionable account by J. Mazza: Seixas requested that Scarlatti give him keyboard lessons and the older master, upon seeing his young organist play, responded humbly that it was he who should receive instruction from Seixas. Little is actually known about the relationship between the two during Scarlatti's eight-year tenure at the Royal Chapel, since the older composer departed for Spain in 1728. It is generally believed, however, that the two shared a mutual respect, though one might surmise that Seixas was the more deferential, since his music did divulge an influence by Scarlatti. Still, Seixas' style had many distinctive features: his keyboard works generally featured simple harmonies and thus a greater focus on thematic content; his sonatas, unlike Scarlatti's, had multiple movements and their structures often exhibited greater complexity, somewhat auguring those of the Classical period. Seixas retained the organist post at the Royal Chapel for the remainder of his life. One outstanding event from late in his career came in 1738 when he was knighted by King João V, a further indication of his talent and prominence.