How is the iMac's display measured?

I am thinking about switching to an iMac for the first time. The new iMac displays come in 20" and 24" sizes. How exactly is that measured? For example, is it 20" from one extreme corner of the display to the diagonally opposed corner? Is it measured straight across the screen at 90-240 degrees? Does the measurement only refer to the actual display, or does it include the aluminum fascia?

Sorry - I live in a place where I can't see one close up, and I am trying to determine how big the display will be on my desk.

Thanks, O. J.