We carry Kiku brand aluminum bonsai wire which has an annodized coating to give it a brown color that blends with the color of the tree's bark. We do not sell copper wire, although that may be used as well. The function of the wire is to allow you to bend and shape the trees' branches. The wire is wrapped around the branch and then bent into the desired position. It is always removed before the tree can grow into it. Aluminum wire has about half the holding power of copper wire which is a disadvantage. However, the advantage of aluminum over copper is that it does not have to be annealed (heated in a fire) before it can be used. In addition, because it is a softer metal, aluminum is less likely to damage the bark on delicate trees. Aluminum wire can also be more easily removed from a tree and reused. The "mm" designation on each wire size is a reference to its thickness in millimeters. 1.0 mm wire is slightly thicker than a human hair. 6.0 mm wire is slightly thinner than a pencil. The thicker the branch, the thicker the wire required to bend it.