You can easily extend your Windows desktop across more than one monitor by plugging two or more monitors into a desktop computer or one or more monitors into a laptop. Most laptops allow you to connect one external monitor. A desktop that spans two or more monitors significantly increases your desktop area so that you can drag windows, program icons, and other items to any location on the extended desktop.
By default, when you connect an external monitor to a laptop, the same image (mirror image) of your desktop appears on the external monitor. Before you can drag a window from your laptop screen to the external screen, you must extend your display by changing your display settings. By contrast, when you connect another monitor to a desktop PC, the display is set to "extended" by default, and you should be able to drag a window from one screen to the other without changing any settings.
To change your display settings to extended
- Open Screen Resolution by clicking the Start button , clicking Control Panel, and then, under Appearance and Personalization, clicking Adjust screen resolution.
- Click the drop-down list next to Multiple displays, click Extend these displays, and then click OK.
The Multiple displays option appears only when Windows detects more than one monitor connected to your PC.
Source Article: Microsoft