From the rail apex to where it joins the plane of the hull/bottom surface. Blue area in pic below.
[IMG]http://i153.photobucket.com/albums/s225/SURFFOILS/revrails.jpg[/IMG]
Ive been making flat alaia/ paipo-style bodyboards with rails like the red ones and I flip the boards over and ride them up-side-down so the rail apex is reversed. Doesnt sound too big a deal but theres something Ive noticed....
A 30/70 or 60/40 feels similar to a standard surfboard rail but flipped, a 70/30 rail feels fine in a straight line until initiating a turn and then theres this huge inertia and loss of lift as the board in angled into the water. The board slows dramatically and the rails buries itself into the wave. Even a 60/40 flipped has a lareg variation of performance.
With the board being the same shape on deck and hull the only diff when flipped is the rails, and its a huge performance variation and I can see how having fat or thin tucked rails in the front and then sharpening them up in the tail would allow the nose to 'fall' into the wave as a turn is begun with the tail holding drive.
As these rails are flat lines I can see that the curve in a rail doesnt hold water but its more about the removal of foam allowing the board to enter the wave, Its pushed in rather than pulled in or held by water wrapping.