In that same year Mido launched watches with unbreakable mainsprings. The first mainsprings were made of steel without tempering or hardening processes. Wrist watches can use muscle power to keep their mainsprings wound up. Large mainsprings in clocks are immobilized by "mainspring clamps" before removal. Clock mainsprings are similar to watch springs, only larger. Broken mainsprings were the largest cause of watch repairs until the 1960s. Since then, the improvements in spring metallurgy mentioned above have made broken mainsprings rare. Mainsprings appeared in the first spring-powered clocks, in 15th-century Europe. Mainsprings usually broke at their attachment to the arbor, where bending stresses are greatest. However, even in their 'let down' state, mainsprings contain dangerous residual tension. Mainsprings are characterised by their "strength" (thickness) and "height" (width).