For garden walls, or such as have no weight to carry, the footings need not be made of so many courses, nor so broad, but every wall must have two courses at least for a foundation. The bricklayer makes use of a string stretched between two pins, to enable him to keep his work straight; and he lays the outermost bricks, those forming the face of the wall, carefully by this guide, setting each brick alternately lengthwise and transversely, and spreading a layer of mortar on the brick beneath, to form a bed for the new one to lie on, and also a layer between each upright joint. It is usual only to lay the outer bricks in this manner, and to fill up the interstices of those forming the interior of the wall by pouring mortar on each course previously laid dry with sufficient interval between them. The workman as he proceeds, repeatedly makes use of his level and square; by the former, he examines whether the face of his wall, and all the corners, or _arrises_, are correctly perpendicular, and whether the courses of bricks are laid horizontal. Apertures, such as windows or doors which are to be formed in the wall, are marked out on the wall when the work is built up to the height where they are to commence; in carrying up the _piers_ between these windows, it will frequently happen that the width of the pier is not precisely commensurate with a certain number of bricks or half-bricks, but that a brick must be cut to bring the work to the correct dimensions. This smaller piece is termed a _closure_, and is usually placed within a brick or two of the arris of the window or door, and preserves its place for the whole height of the pier. The thickness of brick walls is described by the number of bricks’ length they contain in that direction: thus a nine-inch wall is one-brick thick; a brick-and-a-half wall is fourteen inches; a two-brick wall is eighteen inches thick, and so on. The walls of small houses are often only one brick thick, even when they are two stories high; but usually a wall to be steady should decrease in thickness half a brick at least every story, and for a large substantial building of four or five stories, the main walls should be two-and-a-half bricks at least on the basement story, and one-and-a-half at the top; but of course the size of the apartments, or, in fact, the area of wall which is to remain without any lateral support, must govern the strength of it, as well as the total height to which it is to be raised.