The bouldering grade scale common in Europe: from 3 through 6A up to 9A today, with A, B and C sub-steps.
The scale is open-ended and non-linear — the step from 6A to 6B is far smaller than from 8A to 8B. A plus (6A+) marks an intermediate step. Don't confuse it with the American V-scale (V0, V1, …), which maps roughly 6A ≈ V3, nor with the French sport-climbing scale (6a, 7b — lowercase, for roped routes). Worth keeping in mind: gym grades aren't comparable to outdoor grades and vary considerably between gyms. Use grades as rough guidance for picking training, not as a measure of your worth — that's the single biggest source of frustration in the first year.