The French scale: B, TB, TTB, SUP, FDC
The state of preservation accounts for much of the price. France uses a scale of letters, less finely graded than the American seventy-point system, but easier to read.
The grades
B, for beau, marks a heavily worn coin: the type remains identifiable, the legend is partly legible, the relief is flattened. TB, très beau, corresponds to clear but even wear, with the features of the portrait still present. TTB, très très beau, is the grade most often met with in trade: wear touches only the highest relief, and the legend is complete. It is roughly the equivalent of Very Fine.
SUP, superbe, describes a little-circulated coin, with crisp relief and the detail of the hair preserved. SPL and FDC, splendide and fleur de coin, are reserved for pieces practically as they left the die, with no wear from circulation. On third-century coinage they are rare, and priced accordingly.
The refinements
TTB+ or TTB- are written as a matter of course, and mixed states are given with a slash: TTB+/TTB means an obverse better than the reverse. This is useful, because the two faces rarely wear at the same rate: the reverse, struck by the moving die, often receives less metal.
What the grade does not tell you
Two coins graded TTB can look very different. The grade measures wear, not the quality of the strike, not the centring, not the look of the surface. A coin little worn but poorly struck, on a short flan, is worth less than a slightly more circulated example that is complete and well brought up.
Our advice to anyone starting out: look at the portrait first. If it pleases you in an enlarged photograph, it will please you in the hand. The letter that comes with the coin is only a summary.