The heckelphone is a double reed instrument of the oboe family, but with a wider bore and hence a heavier and more penetrating tone. It is pitched an octave below the oboe and furnished with an additional semitone taking its range down to A. It was intended to provide a broad oboe-like sound in the middle register of the swollen orchestrations of the turn of the twentieth century. In the orchestral repertoire it is generally used as the bass of an oboe section incorporating the oboe and the cor anglais (English horn), filling the gap between the oboes and bassoons.
The heckelphone is approximately 1.3 m (4 ft 3 in) in length, and is quite heavy: it rests on the floor, supported by a short metal peg attached to the underside of its bulbous bell. An alternate second bell, called a "muting" bell, is also available, which serves to muffle the instrument for playing in a small ensemble. This arrangement is unique among double-reed instruments. It is played with a large double reed that more closely resembles a bassoon's than an oboe's reed.
Taken from https://en.wikipedia.org/wiki/Heckelphone