Using either annealed copper or aluminum wire to modify and hold the shape of a tree allows the bonsaist to realize the plastic aspect of his art. Cutting and carving constitute the subtractive aspect, while only nature can perform additive operations on bonsai. Other techniques for shaping trees may also be used either separately or in conjunction with wiring. These techniques include tying branches down with string, hanging weights on branches, and the use of various levers or jacks. Wiring provides a general technique for achieving effective branch placement and so is one of the most commonly used techniques..