A+ = EXCEPTIONAL: reserved for a guitar that is extra special in every way.
A = NEW: a new guitar factory fresh + just out of the paint shop. perhaps an older guitar with original spec with little or no use that still looks shop new.
A- = OUTSTANDING: collector standard. all original. not new, but not far off. very rarely played with respect to the age.
B++ = EXCELLENT: virtually pristine such as a shop floor guitar, as good as it gets for most owned/played guitars that have been looked after.
B+ = VERY GOOD: used, original, not pristine but well looked after and used with care.
B = GOOD: used, perhaps not all original but eminently playable
B- = FAIRLY GOOD: maybe a mark or scratch or missing feature that stops it being 'good'.
C+ = ABOVE AVERAGE OVERALL: used, perhaps non original parts and marks/dings but looked after and playable.
C = AVERAGE: with respect to age and expected usage. these are workhorse instruments that have been cared for but well used as working tools.
C- = BELOW AVERAGE OVERALL: a more travelled workhorse, but still definitely playable as is.
D* = FAIR: well used, non original parts, possibly some repairs. Needs a set up or overhaul preferably but just about playable as is.
E* = IN NEED of REPAIR / RENOVATION: forgotten, perhaps neglected or even showing damage. Not playable as is.
F* = RECYCLE / SKIP / ORNAMENT / PROP: beyond hope musically, judge only on its visual or nostalgic appeal, sculptural potential or what it might once have been?
[*] D, E and F can also be further tuned with +/- indications where appropriate to help better establish the cosmetic and structural condition.