You plug an external monitor into your laptop or a second monitor into your desktop computer. At best, nothing happens. At worst, something horrible happens. Find out how to get it right, whether you want to clone your display or spread it across multiple monitors.
Laptops give us the ability to go mobile but usually with a number of compromises compared to a more full-featured desktop machine. The most obvious compromise is screen size. The typical laptop screen is tiny compared to what you probably have sitting on your desk, but setting up your laptop to use an external monitor can give you the best of both worlds.
But multiple screens aren't limited to laptops. If you spend hours in front of a desktop computer every day, a second or even third monitor can help you work more efficiently.
All it takes to add a second monitor is an extra video card, or a video card with multiple outputs and a bit of configuration of Xorg.
First, you need to determine your video chipset, which in turn will determine which drivers
you can use. If you don't know the chipset, you can open
System→Administration→Device Manager, and then scroll down the devices
on the left until you find a device labeled "AGP" or "video."
Alternatively, you can open a terminal and type
lspci to see a list of all PCI devices in your system: look for a line that says "VGA compatible controller" or something similar. If your laptop has an nVidia or Radeon (ATI) video chipset, you have the ...