Powerpoint 2010 - Why 10" x 7 1/2" is the size?

I saw in the design tab/page setup button that the standard size is 10" x 7 1/2".
Why is this?
Why not letter size?
This seems like an arbitrary setting for page size.
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Echo_SConnect With a Mentor Commented:
Because PowerPoint was designed for presentations -- for screen, not for print, and most screens are (or were at one time, anyway) 4x3. 10" x 7.5" is 4x3 ratio.

We're moving into a wide-screen world, though, and PowerPoint 2013 reflects this with its new slide size: 13.333" x 7.5", which is a 16x9 ratio. (Here's a little more on that: http://echosvoice.com/size-matters/ )
brothertruffle880Author Commented:
Thank you.  That was the data I was looking for!
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