The scale length is the distance from the nut to the bridge i.e the length of the open string. The octave mandola appears to be a recent invention to fill the same niche as the tenor banjo. Most instruments in the family have 4 courses as can be seen here on this Gibson mandolin, but there are instruments with 3 or 5 or even 6 courses. The courses (where there are 4) are tuned GDAE respectively. Since the folk revival, similar instruments have been made, but not necessarily given the same names! There is a continuous debate about the names of some members of this family, their characteristics and origins. The main characteristic of the family however is that they all have paired strings (called courses) tuned in unison or octaves.