A box crib is the simplest, most stable and most common method of "cribbing". It is constructed by arranging sets (2 or more) of matched blocks in a regular log-cabin style to form a rising square or rectangular frame. The more blocks on each level, the greater the number of support points and therefore the greater the strength of the crib tower. In trench rescue training materials three basic types of box cribbing are the 4-point, 9-point and full crib. The four point type has two timbers on each level thus four points of contact. Three timbers on each layer makes nine points of contact. The full crib type has each layer filled with timbers. Each point of contact carries a maximum load depending on the size and species of the timbers.