This type of window is the most common in the United States. Two sashes slide up and down past each other in tracks. When open, they are held in place by counterweights, a spring balance, or a friction device. Depending on the type, both sashes may move or just the lower one, in which case it is a single hung window. At best, only 50 percent of the window can be open at any time. Millions of homes across the country still have older double-hung windows that operate with cast iron counterweights and pulleys hidden in the side jambs. The weights vary in length depending on the weight necessary to counterbalance the window. They are separated by a narrow strip of wood called a pendulum to keep them from knocking into each other. If the pulleys stick or the sash cord breaks, the weights must be removed through an access panel in the jamb. Because of theft complexity, these windows are no longer made. Newer double-hung windows operate with a variety of tension spring balances or friction devices.