1. Open pipe organ ranks, usually metal, having a strong fundamental tone with few harmonics. 2. Lowest musical tones in a fairground organ, large orchestrion, etc. A deep bass tone provided by bourdon, violoncello, or other bass-range pipes. Used as accompaniment for other pipes or instruments, never as solo. These bass pipes are sometimes referred to as foundation pipes. * Synonyms: bass division, bass registers, etc.