The viol player Diego Ortiz, who was musical director at the court of the Spanish viceroy in Naples in the mid-16th century, wrote a textbook which became authoritative for the art of improvisation during the Renaissance. This textbook contains pieces with written improvisations in the tradition of the Renaissance period. Although the writing of improvisations was a contradiction in terms even then, Ortiz' pieces are excellent textbooks for the study of the structures and rules of this art.