One of the most important tools on any construction job is a level. This tool is a combined spirit level with a center portion given over to an inclinometer, an instrument which tells you the number of degrees you are off from horizontal. A clear rectangle of glass, which is free to pivot, hangs vertically (the glass has been tinted in this photograph for clarity) and points to the angle of incline. The glass pane maintains vertical by gravity. An adjustable metal hook at the right end of the tool allows the carpenter to snag a beam and let the tool rest on the beam without sliding down. Modern spirit levels are made out of aluminum or plastic and replace beautiful wood and iron designs of the late 19th and early 20th centuries. . Horizontal is indicated with a small glass tube filled with alcohol that contains a tiny air bubble; the bubble is centered in the tube when the level is held perfectly horizontal. This is an intermediate design, later than the plumb bob, but before the spirit level became universal. Get a closer view of the object (55KB).