Bricklayers work in a highly skilful trade, and require an ability to work consistantly and to a precise standard. Bricks are laid in a wide variety of patterns, known as 'bonds', which give the brickwork the strength and durability, as well as providing the heat-and-cold insulation properties required for the building. The final, and also highly skilled, work is to 'point' the brickwork, which is the filling of the gaps between the outer edge of the bricks with a neat and regular layer of waterproof filling material. Bricklayers mainly erect the brick structure of buildings, cellars and façades, weight-bearing structures and partition walls, chimneys, stairs, balcony banisters. They use either bricks and mortar or concrete: sometimes they will apply a finishing surface to the brickwork.