Now mainly replaced by the Spirit Level but still used when a high degree of accuracy is required. The 125mm x 25mm wooden rule can be up to 2.00m long; the lead, or brass, plumb bob is held centrally from the top of the rule and can move freely in an opening at the lower end. It is read against a gauge line down the centre of the rule. Also, made with one side vertical and one side inclined; this battering plumb rule, read in the normal way, is used when one wall face is inclined inwards, e.g. retaining walls and buttresses.