When a body is freely suspended by a string and is at rest the force of gravity on it, acting vertically downwards, is balanced by an equal and opposite force or tension in the string. Being flexible, the string therefore sets in a vertical direction. This is the principle of the plumbline, which consists of a small leaden bob supported by a thin cord. From the earliest times, plumblines and plumbrules (Fig. 6.6) have been used by builders for the purpose of testing the uprightness of walls, pillars, and so on during the course of construction.