vocal exercises sung to the solmization syllables (do, re, mi, etc.) and, by extension, vocalizes, or exercises sung to a single vowel, often florid and difficult to master. Solfeggio collections survive from the 17th century onward, with examples by leading composers of 18th-century opera, such as Nicola Porpora (also a singer and famed singing teacher) and Alessandro Scarlatti and, reaching into the 19th century, Luigi Cherubini. Later composers of such exercises include Maurice Ravel, Gabriel Fauré, Vincent d'Indy, Ralph Vaughan Williams, Heitor Villa-Lobos, and Sergey Rachmaninoff. The word solfège sometimes refers to an intensive course in the knowledge of musical intervals and their notation.