The octave mandolin is a whole step lower than a standard mandolin.  Having come back into popular culture in the past decade this unique instrument finds itself useful in many different types of music styles.  Whether used as an accent in solo portions of songs or used to accompany the singer with a harmonious chordal progression.  You may never have heard of it, but most will enjoy its deep droning tonal texture.    The body shape that I use for my Octave is a downscaled arch-top guitar shape similar to a Gibson L-5.  This shape gives the instrument enough air space for the bass stings to produce deep rich sustain that sets it apart from is half sized sibling.