Sliding windows operate from side to side. They are normally referred to as XO, OX, XX or XOX. The XO, OX and XOX, are best described as a widow with 1 stationary panel (O) and a moving panel (X). The XX window is a window where both panels move independent of one another. These styles of windows are the most common in residential building due to their ease of operation and the size the windows can be manufactured in larger sizes than hung windows. Sliding windows role on wheels, as opposed to hung windows which require a spring mechanism to help with the operation of the window.