In construction and architecture, a slab[1]​ is the lower horizontal member of a wall or building to which inclined members are attached, horizontal "Streak (architecture)") or vertical "Stile (construction)"). The slabs are generally made of wood, but currently they can be made of any material. The wood at the top of a wall, panel or frame is also called a run (architectural) run, top plate, top sill, sleeper, wall plate or tie sill, depending on its position or function. On the other hand, the upper sill of an opening is called a lintel, and the lower sill is called a pedestal if it is a window or a threshold if it is a door.[2]​.

In historic buildings, the sills were almost always large, solid timbers framed in the corners. They had to support the reinforcement "Armor (construction)") and their uprights "Stand (construction)") were placed directly on the stone or brick foundations, pillars or piles (wooden posts nailed or placed in the ground). In these constructions, it could be said that the slabs had a racing function "Carrera (architecture)"), they supported the weight of the walls (posts and studs "Montante (construction)"), as well as the floor joists.

Although uncommon, there are examples of historic buildings in the U.S. where the floor joists rest directly on the foundation, and a plank or wooden platform placed on top of them acts as a sill.[3] Another unusual historic construction technique is one in which the posts or studs of a wood-frame building rest directly on the foundation or directly on the ground and the sills are wedged between the posts, called interrupted sills.

In modern wood construction, sills generally come in sizes of 2x4, 2x6, 2x8, and 2x10. In the light timber frame structure, the sills are made of treated wood. The frames or panels are placed over the sill plate, which is usually anchored to the subfloor, often with J-bolts, to prevent the building from moving off the foundation during a severe storm or earthquake.[4] Building codes require that the bottom of this sill be kept 6 to 8 inches above the finished grade, to prevent termites or the sill from rotting. In addition to the basal sill, the framework frames with platforms have a lower sill, upper sill and mooring sill.[4]​[5]​.