The objective here is to explain how to work with multiple displays connected to a single computer. The focus is on Microsoft Windows, eventually I will extend this to other operating systems.
Prior to Windows XP this was not possible without specialist Display Adapter software. So, I'll skip that and move straight to Windows XP and beyond.
A few years ago I did struggle to get a friend's Sony laptop to work with a projector, it turned out that it would not enable the external display until it had mains power. Very odd but mentioned just in case you have a similar issue.
- turn the projector on and wait for it to warm up and be ready to go
- connect your laptop to the projector. Usually this is via an SVGA cable with a 15-pin D shaped connector but DVI, Display Port or HDMI should work just as well
- some laptops will detect the external display and automatically "duplicate" the laptop screen onto the projector
- press Win+P (that's the "Windows" key in the bottom left of your keyboard), then use the right and left arrow keys to select Extend
- your desktop should now be extended over two displays, your laptop and the projector
- in addition, right click desktop and select "Screen resolution", this allows you to keep your laptop screen at it's "native" resolution and do the same for the projector, even if one is 1920x1200 and the other 1024x768
- plug in projector
- some laptops will detect and automatically "duplicate"
- right click on your desktop and select "Properties"