Originally Posted by
Delmarva
Very informative post. The only thing I would add is that the top tube has changed orientation over time from being largely parallel to the ground to angling downward toward the seat post. So the traditional diamond or rhombus has become much flatter. At least one bike dealer says this was done to reduce costs by reducing the number of bike sizes manufactured. Or to allow a given bike size fit a wider range of riders.
I think the dealer is probably right. The "small, medium, large" deal started to supplant the "49 through 63 in one cm. increments" system around the time that carbon monocoque frames became popular. I understand the molds for such frames can run a quarter million dollars apiece, so while the manufacturers might claim that they're just trying produce a more adaptable frame, I imagine reducing startup costs by a few million dollars has something to do with it.
Having said that, the "small, medium, large" thing seems to work okay for most folks. I just can't imagine a "boutique" builder of classic lugged steel framesets trying to get away with it.