Wagner's "Der Ring des Nibelungen" (1876) employed the contrabass trombone for the first time in the opera house. He had a horn with double slides built in 18’ B♭ (without a valve) in Berlin, by C.A. Moritz. This horn had only 6 positions, and the low E1 called for in "Der Ring des Nibelungen" was only possible by lipping down. This type of contrabass trombone has lasted into the 20th century, and is complemented by a valve which changes the pitch of the horn to F1. The double-slide contrabass trombone has less resistance than a tuba, but takes more air to produce a tone, and despite its modern innovations over the Renaissance horns, it remains somewhat taxing to play.