|System preferences - Display - Arrangement|
I know a lot of you will be saying "Well, Duh!" but if you don't know, you don't know :-)
Hopefully this will help someone else.
If you connect a second or third monitor to your Mac occasionally the "Primary" monitor (ie the one with the toolbar and the dock) isn't on the display that you wanted. I have usually fixed this in the past by plugging and unplugging monitors until I got what I wanted :-)
However, the right way to do it is this:
Go to System Preferences,
Select the "Arrangement" tab (this won't be there unless you have at least two monitors connected)
If you look at the image above you can see the arrangement menu.
You can drag the blue boxes to match the physical position of your monitors, but you can also drag the white line that you see at the top of the box to the monitor that you would like to be the "primary" monitor.
Simple isn't it?
I've been using Macs for years and I never knew. Doh!! :-)