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Windows supports a single multi-monitor mode, usually referred to as standard or extended desktop mode, also DualView or independent displays mode. In this mode, all monitors connected to the installed video cards form a single desktop, you can move the mouse and applications to any monitor.
Each monitor can use different settings (resolution, color depth and refresh rate). Most multi-monitor video cards (video cards which can drive 2 or more monitors) support additional, video card-specific modes.
Very common is clone mode
: in this mode, the same image is shown on 2 or more monitors. Clone mode is usually limited to the monitors connected to a single video card, for example you couldn't clone monitor 1 (on video card 1) on monitor 3 (on video card 2) if you have two dualhead cards installed.Span mode
(also called stretched mode): in this mode, all the monitors connected to a single video card form a single large monitor. Windows thinks that you are using a single monitor instead of 2 or more, and each monitor needs to use the same resolution and color depth settings, and usually also the same refresh rate.
This mode is mainly useful for forcing applications which have no native multi-monitor support to use all available monitors. For example most games will only run on the primary monitor, and in span mode all monitors form a single large primary monitor.
When using span mode, Display Properties will usually show the primary monitor running at a widescreen resolution, for example 2048x768 (2 monitors at 1024x768 each), with the secondary monitor disabled.
To enable video card-specific multi-monitor modes, you usually need to disable the secondary monitor(s) connected to the video card, then open advanced display properties for the primary monitor, select the video card manufacturer's custom settings tab and select the multi-monitor mode you want to use. For more on the multi-monitor modes supported by the various video card manufacturers, take a look at one of the following reviews: ATI review
, Matrox review
, Nvidia review