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add some introduction, not ready yet

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= Introduction =
The most important concept in understanding how X deals with multiple monitors is the X protocol screen. This page refers to it as SCREEN. Traditionally in X11, a SCREEN had a one-to-one correspondence with a physical display device (a monitor). Dual- or multi-head graphics cards did not really exist, and things were simple. The X configuration file ({{{xorg.conf}}}) has one {{{Section "Screen"}}} for each SCREEN, either explicitly written in the file, or nowadays implicitly created by the X server.
SCREENs are independent. They can have, not only different video modes, but also different color depths and resolution (dots-per-inch, used in scaling e.g. true-type fonts). Windows can never span more than one SCREEN, nor can they be moved from one SCREEN to another. The only way (that your author knows about) to switch focus from one SCREEN to another is with the mouse. Internally, each SCREEN is driven by a separate driver (DDX) instance, since SCREENs had one-to-one correspondence with graphics cards, too.
Add a dual-head graphics card. Suddenly there is a single card with two physical monitors, and things get complicated. One solution for a driver (DDX) is to create one SCREEN per head, which is called Zaphod mode (after Zaphod Beeblebrox, from the Hitchhiker's Guide to the Galaxy). Another solution is to pretend that there is only one monitor, and use just one SCREEN, which is what the Nvidia TwinView mode does. The third and the only proper way to deal with it is the Randr extension, which is a relatively new invention. Randr exposes the dual-head card as a single SCREEN, yet having a standard way of managing the multiple monitors.
Unfortunately, Randr does not help with multiple graphics cards, yet. Each card is forced to be a separate SCREEN. However, the Xinerama feature changes that. (Xinerama is an older invention than Randr.)
= Multi-monitor desktop with Nouveau =
If you have a single GPU with multiple heads, it should all just work for you with [[Randr12|RandR 1.2]] and offer full (whatever is implemented) graphics acceleration. This page describes the situation for multiple GPUs, which considers both multiple cards and multi-GPU cards.
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