A weight machine is an exercise machine used for weight training that uses gravity as the primary source of resistance and a combination of simple machines to convey that resistance to the person using the machine. Each of the simple machines (pulley, lever, wheel, incline) changes the mechanical advantage of the overall machine relative to the weight. Stack machines A stack machine—also called a stack or rack—has a set of rectangular plates that are pierced by a vertical bar which has holes drilled in it to accept a pin. Each of the plates has a channel on its underside (or a hole through the middle, as visible in the picture) that aligns with one of the holes. When the pin is inserted through the channel into the hole, all of the plates above the pin rest upon it, and are lifted when the bar rises. The plates below do not rise. This allows the same machine to provide several levels of resistance over the same range of motion with an adjustment that requires very little force to accomplish in itself. The means of lifting the bar varies. Some machines have a roller at the top of the bar that sits on a lever. When the lever is raised the bar can go up and the roller moves along the lever, allowing the bar to stay vertical. On some machines the bar is attached to a hinge on the lever, which causes swaying in the bar and the plates as the lever goes up and down. On other machines the bar is attached to a cable or belt, which runs through pulleys or over a wheel. The other end of the cable will either be a handle or strap that the user holds or wraps around some body part, or will be attached to a lever, adding further simple machines to the mechanical chain. Usually, each plate is marked with a number. On some machines these numbers give the actual weight of the plate and those above it. On some, the number gives the force at the user's actuation point with the machine. And on some machines the number is simply an index counting the number of plates being lifted. The early Nautilus machines were a combination of lever and cable machines. They also had optional, fixed elements such as a chinning bar. Universal Gym Equipment pioneered the multi-station style of