The subcontrabass saxophone is a type of saxophone

Saxophone

The saxophone is a conical-bore transposing musical instrument that is a member of the woodwind family. Saxophones are usually made of brass and played with a single-reed mouthpiece similar to that of the clarinet. The saxophone was invented by the Belgian instrument maker Adolphe Sax in 1846...

 that Adolphe Sax

Adolphe Sax

Antoine-Joseph "Adolphe" Sax was a Belgian musical instrument designer and musician who played the flute and clarinet, and is best known for having invented the saxophone.-Biography:...

 patented and planned to build but never constructed. Sax called this imagined instrument saxophone bourdon (named after the lowest stop on the pipe organ

Pipe organ

The pipe organ is a musical instrument that produces sound by driving pressurized air through pipes selected via a keyboard. Because each organ pipe produces a single pitch, the pipes are provided in sets called ranks, each of which has a common timbre and volume throughout the keyboard compass...

). It would have been a transposing instrument

Transposing instrument

A transposing instrument is a musical instrument for which written notes are read at a pitch different from the corresponding concert pitch, which a non-transposing instrument, such as a piano, would play. Playing a written C on a transposing instrument will produce a note other than concert C...

 pitched in B, one octave

Octave

In music, an octave is the interval between one musical pitch and another with half or double its frequency. The octave relationship is a natural phenomenon that has been referred to as the "basic miracle of music", the use of which is "common in most musical systems"...

 below the bass saxophone

Bass saxophone

The bass saxophone is the second largest member of the saxophone family. Its design is similar to that of the baritone saxophone, with a loop of tubing near the mouthpiece. It was the first type of saxophone presented to the public, when Adolphe Sax exhibited a bass saxophone in C at an exhibition...

 and two octaves below the tenor saxophone

Tenor saxophone

The tenor saxophone is a medium-sized member of the saxophone family, a group of instruments invented by Adolphe Sax in the 1840s. The tenor, with the alto, are the two most common types of saxophones. The tenor is pitched in the key of B, and written as a transposing instrument in the treble...

.
Until 1999, no genuine, playable subcontrabass saxophones were made, though at least two gigantic saxophones seem to have been built solely for show.