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Ra (arithmetical mean roughness) is the most commonly used surface roughness parameter. It represents the average absolute deviation of the profile from the mean line, measured in micrometers (μm). Lower Ra values mean smoother surfaces.

MRR stands for Material Removal Required. The plain basic V-symbol means "any process allowed". A bar across the V means "machining required (material removal required)". A circle on the V means "no material removal allowed" (surface left as-produced).

Lay direction symbols indicate the predominant surface pattern direction:

• = Parallel to projection plane
• ⊥ Perpendicular
• X Crosshatch
• M Multi-directional
• C Circular
• R Radial
• P Non-directional/particulate

ISO 1302 defines how to indicate surface texture on technical drawings (symbols and notation). ISO 21920 defines the actual profile parameters (Ra, Rz, Rq, etc.) and their measurement methods. They are complementary standards.

Typical values: Ra 0.1–0.4 μm for bearing surfaces and seals; Ra 0.8–1.6 μm for mating/sliding surfaces; Ra 3.2–6.3 μm for general machined surfaces; Ra 12.5–25 μm for rough machining and castings.