An application will normally have one main window and may have one or more child windows, subordinate to this main window. The main window is known as the parent of these child windows, and the child windows themselves may be the parents of other child windows. An application may thus have a hierarchy of windows. Too deep a hierarchy, however, can be very confusing to the user. Windows which exist at the same level in the hierarchy and have the same parent window are known as siblings.
While multiple windows may be visible on the screen at any time, the keyboard can only be associated with one window at a time; this window is said to have "input focus". Input focus is normally attributed to a control window on the client area, such as an entry field or list box. The window that contains the control with input focus, is then called the "active window". Presentation Manager automatically indicates this by making the active window the top window and changing the colors of its sizing border and title bar. The active window and input focus can be easily changed by the user. The same technique is used for both windows and controls; move the mouse over the desired window and select it using mouse button 1. Alternatively, a user may key combinations, such as ALT+ESC or ALT+TAB, to move between windows.