The Windows logo key (also known as Windows key, start key, logo key, flag key, super key, command key or flag, home) is a keyboard key which was originally introduced on the Microsoft Natural keyboard in 1994. This key became a standard key on PC keyboards. Tapping this key invokes the operating system's start menu, if it has one. In Windows, Ctrl+Esc performs the same function, in case the keyboard lacks this key. Historically, the addition of two Windows keys and a menu key marked the change from the 101/102-key to 104/105-key layout for PC keyboards: compared to the former layout, a Windows key was placed between the left Ctrl and the left Alt; another Windows key and—immediately to its right—a menu key were placed between the AltGr (or right Alt key on keyboards that lack AltGr) and the right control key. In laptop and other compact keyboards it is common to have just one Windows key (usually on the left). Also, on Microsoft's Entertainment Desktop sets (designed for Windows Vista), the Windows key is in the middle of the keyboard, below all other keys (where the user's thumbs rest). On Windows 8 tablet computers, hardware certification requirements initially mandated that the Windows key is centered on the bezel below the screen, except on a convertible laptop, where the button is allowed to be off-center in a tablet configuration. This requirement was relaxed in Windows 8.1, allowing the Windows key to be placed on any bezel or edge of the unit, though a centered location along the bottom bezel is still preferred.