H.M.S. Pinafore, or The Lass that Love a Sailor, is a comic opera in two acts, with music by Arthur Sullivan and libretto by W. S. Gilbert. It is one of the Savoy Operas, and the first big hit by Gilbert and Sullivan. It opened at the Opera Comique in London on May 25 1878 for a run of 571 performances, which was the second longest run of any musical theatre piece up to that time (after the operetta Les Cloches de Corneville). H.M.S. Pinafore was Gilbert and Sullivan's fourth operatic collaboration.