The size most commonly used in the orchestra is called the contrabass-tuba, or double-bass tuba.

NPR: Young Tuba Player Gets Nod from Phila. Orchestra

But on the contrabass trombone, you get this huge wall of sound .

WSJ: It Takes Brass to Play the Wagner Tuba | By Barbara Jepson | The Ring of the Nibelung

Yet to principal bass trombone David Ridge, who has played contrabass trombone for San Francisco Opera "Ring" cycles, the advantages of following Wagner's specifications are significant.

WSJ: It Takes Brass to Play the Wagner Tuba | By Barbara Jepson | The Ring of the Nibelung

In the "Ring, " four Wagner tubas join an enlarged brass section that includes other orchestral rarities like the bass trumpet, contrabass trombone and contrabass tuba, creating "choirs" of gleaming brass.

WSJ: It Takes Brass to Play the Wagner Tuba | By Barbara Jepson | The Ring of the Nibelung