Whether the devices are to be placed indoors or outdoors, the warning devices (the “studs”) must unambiguously materialize the proximity of an imminent hazard. The profile, width and height  of the studs therefore meet strict standardized requirements.

In many countries, including Belgium, the standard NF-P98 351 is used, in which the studs are arranged orthogonally with respect to each other with a center to center distance “S” (see diagram opposite) of 75 mm. With this scheme one counts about 356 studs/m².