On Mon, 29 Feb 2016 08:59:45 -0800 (PST), TimR
wrote:

On Monday, February 29, 2016 at 10:36:32 AM UTC-5, Don Y wrote:
On 2/29/2016 8:10 AM, TimR wrote:
Take your tape to Home Depot or Lowes. What size are those doors? (the
ones for sale, not the ones you walk through at the front of the store)

That will tell me what they *sell* but won't explain why the two
doors I observed (different homes, different widths, different
construction techniques) are "off".

If all doors are 35.5, then you have determined that a nominal 36 door is actually 35.5.

I find it unlikely a custom door is a half inch different.

Modern doors are always about 1/2" narrower than their listed size.
There has to be some gap around the door, and that 1/2" is your gap. The
INSIDE of the frame is the listed size (32" 34" 36", etc).

If you're replacing an entire door and frame, you need the OUTSIDE
measurements of the frame, an you make your rough opening about 3/4"
larger, so you can align the frame and apply the shims.

Some OLD doors were the actual listed size, but not always....
(Of course back then, the frames had to be 1/2" bigger than the listed
door size).