The anti-slip value of a surface is indicated with an R-value. This R-value is divided into five categories, related to the level of the slip risk. To determine the R-value, the surface is tested on an adjustable, inclined surface. R11 is recommended for surfaces with a normal risk of slipping. Surfaces with an increased risk of slipping should be at least R12. The coefficient of friction is indicated by the μ (Mu) symbol. The NEN7909 describes a method for determining the dynamic friction coefficient of floors and sets limit values for dry floors (μ > 0.30), wet floors (μ > 0.40) and wet floors in swimming pools (μ > 0.45).