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Translations:Xorg/Guide/64/en
From Gentoo Wiki (test)
If the screen resolution looks to be wrong, you will need to check two sections in your xorg.conf.d configuration. First of all, you have the Screen section which lists the resolutions that your X server will run at. This section might not list any resolutions at all. If this is the case, Xorg will estimate the resolutions based on the information in the second section, Monitor.