In order to know how to replace a sash cord you need to understand the basics of a double hung window. A double hung window has two sashes one on top and one on bottom. A sash is the framework that holds the glass. The sashes slide up and down within a window frame. The side frames are the jambs. Heavy metal sash weights connect to sashes with a rope-and-pulley system hidden behind the jambs. The weights provide a counterbalance that makes the sash easy to open.