The heckelphone is of conical bore (wider than the baritone oboe's) and has a bent metal crook and a wooden bell. It is usually built in C an octave below the oboe, with an extended lower register. It was first used by Richard Strauss in his operas Salome and Elektra. The compass is from the second A below middle C to the second G above. Other forms are the smaller terzheckelphone in E♭ and the piccolo-heckelphone in F.