When you plug a projector (or second monitor) into your Macintosh laptop or computer for the first time it will probably display a different screen to your main display. To see the same on both screens you ned to turn on ‘mirroring’ in the displays preferences.
By default the layout of the two screens will be something like this:
In other words the projector is making your desktop area bigger. This is great if you want to have two monitors on your desk because it will give you more ‘desktop’ space. But you may want your projector to display exactly the same as you see on your laptop. Like this:
This is called ‘mirroring’ your display. Your screen and the projector display exactly the same thing.
Do to this you need to turn on Mirroring.
To turn on mirroring you need to go to “system preferences” from the Apple menu on the top left of your OS X screen, and then click on the ‘displays’ icon.
Now click on the “Arrangement’ tab.
Now down the bottom click on the checkbox that says ‘Mirror Displays’.
You will now see the same desktop on your computer screen and projector.