The Font scale (or "Fontainebleau scale") is the bouldering grade system that originated in the famous Fontainebleau forest south of Paris — one of the world's most important bouldering destinations. It is the dominant scale in France, Belgium, Spain, Italy, and parts of the Netherlands. It uses a number plus a letter grade: 4, 5, 6A, 6A+, 6B, 6B+, 6C, 6C+, 7A, 7A+, 7B, and so on up to 9A (the current world maximum).
The Font scale is sometimes confused with the French sport climbing grade scale (which uses similar notation: 5a, 6a, 7a, etc.) — but they are distinct systems for different disciplines. Font is for bouldering only; the French sport scale is for roped climbing.
Rough conversions to the V scale: 5 ≈ V0, 6A ≈ V3, 6C ≈ V5, 7A ≈ V6, 7B ≈ V8, 7C ≈ V9, 8A ≈ V11, 8B+ ≈ V14, 9A ≈ V17. Like the V scale, Font grades are community-derived and somewhat subjective. Outdoor Font grades tend to feel harder than indoor V grades at the same notional difficulty because of the variable rock conditions, stress of being outside, and traditionally tougher Fontainebleau grading culture.