Problem scenario
You have a Mac laptop. You have connected a separate monitor. To get your mouse one the screen, you have to move the direction of the mouse in a different direction relative to the physical location of where you see this external monitor. You have to commit to memory where to move the mouse. You want the external monitor to be mouse-reachable in the direction that it is in front of you. How can you get the position to match?
Solution
Go to the Apple symbol in the upper-right hand corner. Go to System Preferences -> Displays. Click on "Gather Windows." Go to Arrangement. Click on the blue rectangles to identify which monitor is which. Move the rectangles in a way that reflects their physical location.