I am from the analog days and have this *question* arising from my pre-occupation with the discontinued Kodachrome 135 format from of old. Never did got an answer to this question, really. Now that the Internet is generoysly available and knowledgeable, here goes.
Majority of DSLRs use the APS-C format which is based on the (reduced) 35mm format. The professional models uses full frame which is equal to the 35mm format. 35mm format got its root from 70mm film format (see http://en.wikipedia.org/wiki/135_film).
How do you explain the dimension of 24mm by 36mm full frame (whether digital or analog) when the label was always 35mm? I doubt the 36 refers to the 35 exposures at that time.
Note: In my search to have a completely clear understanding of the Nikon's DX/FX format against that offered by Canon, I got driven back to this nagging question.