A spirit-level is important for some work, but not often necessary for the beginner, as a substitute can easily be made. A horizontal or level line being at right angles with a vertical line, a home-made level can be made by using the principle of the plumb-line, as shown on page 96. When the plumb-line hangs freely on the line ab, which is at right angles to cd, the latter line {cd) must of course be level. The frame should be several feet long for levelling large work, as it can be adjusted more accurately than if small.