The 9 extra sub-bass keys of the Bösendorfer Imperial 290 are unique to the make and model. Not only do they provide an extended range, they also resonate sympathetically with the rest of the trings, lendng a richness and warmth to the piano as a whole. Further to this, the design of the Imperial is such that the standard low notes of the piano are positioned towards the centre of the bass bridge, where the sound is most effectively transferred to the piano's extra-large soundboard.