Originally Posted by
LeSexyFishorse
I think they might have been referring to center to top of seat tube since I told them I was looking for a vintage road bike frame.
You also have to be wary that whoever's selling the '51cm' measured frame, whether they just measured the actual distance from the BB to the top of the seat tube. Of course a bike with a sloping top tube will have a much shorter actual seat tube, while you're fitter was likely referencing a size based on a horizontal top tube. IOW, there is a possibility that a sloping top tubed bike might have an actual measurement of 51cm if the seller is just measuring with a yardstick, but be equivalent to eg. a nominally labelled 56cm bike.