Please bear with me guys.....another stupid question.

I come from more of a mountain biking background and with the advent of sloping top tubes and in some cases virtually no top tube,(think Trek Y bike and similar designs) frame sizes got a little more difficult to measure.
Different manufacturers would come up with varying places to measure from to come up with "effective" seat tube sizes.

So for my Prologue, is the seat tube measured from center of bottom bracket to top of top tube or center of top tube or some other combination?

thanks
Matt