1901. The rule-joint ladder (fig. 508.) is used for working on curvilinear roofs either of glass, or domes of lead, stone, &c., which require panes renewed or trees nailed. Each step or foot-board has what is called a stop, to prevent the feet from breaking the glass, and at every joint is a movable foot to project in the case of training trees on such surfaces, in order that their leaves, &c., may not be injured. Such ladders are particularly useful for repairing curvilinear hothouse roofs.