: These are "International Tolerance" grades. Lower numbers mean higher precision (e.g., IT5 is for fine gauges, while IT11 is for rough machining). 2. Types of "Fits" (The Relationship)
The ISO 286 standard, titled defines the international system for tolerances, deviations, and fits used in engineering and manufacturing. It is divided into two primary parts: Part 1: Basis of Tolerances, Deviations, and Fits iso 286 pdf