GDAD is very popular, specially among octave mandolin players. Tom Napper tunes his octave mandolin thus (and manages to play tunes incredibly well in spite of the need to slip up to second position on the top course, something he does automatically on the OM but not on the Tenor Banjo), as do Phil Beer and Steve Knightly (though, confusingly, they call their octave mandolins mandocellos - or in Steve's case 'cello mandolins'). (I also have an Oddy MC like theirs, but tune mine ADGC, making it technically a four string twin course guitar - or, as Tom calls it, a picolo mandobass).

Bouzoukis seem to appear in so many different tunings these days that it's almost impossible to call anything 'Bouzouki tuning' IMO. GDAD is quite common - but does that not make them actually OMs if thus tuned?

The question is; is it the shape of the body that defines the name, or the presence of octave strings (James Fagan, for example, uses no octave strings), or - as I would have it - the tuning, on the basis that this tells you what gauge of strings to buy, and others what you'll be able to play on it.

Tom