The Hypotenuse?

I use a G5 (Mac). I’m amazed by its stability but my background was web programming so I maintain (for sport) other boxes on my little home network; I have linux and windows machines as well. John gave Isa an IBook for school this year (his old one). It’s a decent thing, but it clearly needs a new battery. I went on apple.com to buy a battery and two sizes of IBook came up in the battery department: 12″ and 14″ screens.
I did what I thought was logical, I measured the screen:

Which you can see, is 11 inches and change. Is this the 12″ screen then? So I measure the entire back of the unit:

Which is clearly a little more than 12″. So what is this screen? 12″ right? Well, it turns out that’s wrong! This is the 14 inch model. I know this because I ordered the 12 inch battery and it was too small and opened up the battery to see what was going on. As a non mac user, I didn’t know you could actually see this on the battery itself (14 inch battery) because no PC will tell you anything about itself- particularly it’s screen size from the battery. Not logical.

I complained to John, and he suggested perhaps Macs are doing the hypotenuse,

and he was right. I guess I don’t think like a mac user. This just seemed intuitive to John how you’d measure a screen.