The window sash is the panel of material that holds the window glass and sits inside the window frame. In double-hung windows, both sashes slide up and down in the frame. In casement windows, the sash swings outward as the window opens. While the sash is an often-overlooked part of a window, it has several important roles, and therefore you should put some thought into the sashes on the replacement windows you choose for your Michigan home. Here's a closer look at this important window component.