A menu bar is a graphical control element which contains drop-down menus.
The menu bar's purpose is to supply a common housing for window- or application-specific menus which provide access to such functions as opening files, interacting with an application, or displaying help documentation or manuals. Menu bars are typically present in graphical user interfaces that display documents and representations of files in windows and windowing systems but menus can be used as well in command line interface programs like text editors or file managers where drop-down menu is activated with a shortcut or combination key.
Implementations
Through the evolution of user interfaces, the menu bar has been implemented in different ways by different user interfaces and application programs.
Macintosh
In the Macintosh operating systems, the menu bar is a horizontal "bar" anchored to the top of the screen. In macOS, the left side contains the Apple menu, the Application menu (its name will match the name of the current application) and the currently focused application's menus (e.g. File, Edit, View, Window, Help). On the right side, it contains menu extras (for example the system clock, volume control, and the Fast user switching menu (if enabled) and the Spotlight icon. All of these menu extras can be moved horizontally by command-clicking and dragging left or right. If an icon is dragged and dropped vertically it will disappear with a puff of smoke, much like the icons in the dock. In the Classic Mac OS (versions 7 through 9), the right side contains the application menu, allowing the user to switch between open applications. In Mac OS 8.5 and later, the menu can be dragged downwards, which would cause it to be represented on screen as a floating palette.
There is only one menu bar, so the application menus displayed are those of the application that is currently focused. Therefore, for example, if the System Preferences application is focused, its menus are in the menu bar, and if the user clicks on the Desktop which is a part of the Finder application, the menu bar will then display the Finder menus.
Apple experiments in GUI design for the Lisa project initially used multiple menu bars anchored to the bottom of windows, but this was quickly dropped in favor of the current arrangement,[1] as it proved slower to use (in accordance with Fitts's law). The idea of separate menus in each window or document was later implemented in Microsoft Windows and is the default representation in most Linux desktop environments.
Even before the advent of the Macintosh, the universal graphical menu bar appeared in the Apple Lisa in 1983. It has been a feature of all versions of the Classic Mac OS since the first Macintosh was released in 1984, and is still used today in macOS.