Mandocellos, Octave Mandolins and Related
The Octave Mandolin and its First Cousin, the Octave Mando-Guitar
The Octave Mandolin is a mandolin family version of a tenor guitar or tenor banjo that is tuned in the Celtic G-D-A-E tuning (in other words, an octave lower than a mandolin). The only important differentiator from the tenor guitar is that, instead of four individual strings, an octave mandolin has four (occassionaly five) pairs (called "courses") of strings. Each string in a pair is tuned in unison. A related instrument is often called the "Irish Bouzouki", on which the two (sometimes three) lower pairs are tuned in octaves (a low and high note) like a 12-string guitar. The higher pitched strings are tuned in unison, again, like a 12-string guitar).
Whether the instrument is called an Octave Mandolin or an Octave Mando-Guitar is somewhat academic. If the body looks more like a mandolin, we call it an Octave Mandolin. If the body is more guitar-like (a prevailing trend for contemporary instruments in this class), we called it an Octave Mando-Guitar. Other names have been used for the later type, such as "Octar".
A mandocello is to a bowed ‘cello what a mandolin is to a violin. The mandolin is tuned like a violin and a mandocello is a large mandolin tuned like a ‘cello. The primary differences, of course, is that mandolin family instruments are played with a plectrum rather than a bow and they have twice as many strings…actually four, or sometimes, five pairs (called courses) of strings, each pair being tuned in unison. These instruments are seeing increasing use in Celtic music, Progressive Bluegrass and In Alternative Old-Time (aka "Neo Old-Time", "Old-Time Punk" genres.)