Originally Posted by Tourist in MSN

That is correct, nobody knows.

But if they already have a bike that fits them well, they have something to go by. The last three bikes that I bought (one as a complete bike, two as bare frames), I first looked at effective top tube on the manufacturer's geometry chart. I already had a couple bikes that fit me well, so I had base measurements to work with. Thus it was pretty easy to pick the right size. All three of these were ordered on-line. I first looked at effective top tube length or if that was missing on their chart, the actual top tube length. And then a quick comparison for standover height and bingo, I have the size I need.

Go back a decade and a half ago, I was trying to figure out the size I needed from Thorn by providing various body measurements to them. They decided I needed a size 610S. If this was still the world of "fist full of seat post" sizing, I would have said no to that size because the last horizontal top tube bike that I bought based solely on stand over height was in 1973 when I bought a Raleigh Grand Prix, size 23.5 inch (which today is 59cm). Thus, I knew that with a horizontal top tube, anything bigger than 59 would be too big for stand over. But the Thorn bike I was looking at that they said I need a 61 effective seat tube length, the frame had a sloping top tube and the other measurements on the chart gave me the trust that stand over height would be ok. That bike fits me just fine.

I mentioned earlier in this thread that my rando bike had too long of a reach, changed the handlebars earlier this year. That is one of the three that I bought on line based on top tube length. In this case, when I first built it up, I apparently measured the reach of the handlebars that I installed wrong, that is why I got my reach too long on that bike, a simple case of user error that I finally rectified this year. It was off by less than an inch, so fixing it was not urgent.

And the complete bike that I recently bought, I knew when I ordered it that I would need a different stem to adjust it for size.

You have a valid point that if someone does not have a well fitting bike to use for base measurements, you have to start from scratch. But my point is that do not just size only by seat tube length.