This exchange has been a bit like the old Abbott and Costello "Who's on first?" routine.
I am surprised to hear that chainstay length is measured parallel to the ground. As a number of correspondents noted, this is not really chainstay length at all, but rather the length of the leg of a right triangle. Why wouldn't the chainstay length measurement be the actual length of the chainstay (assuming everyone could agree on the location of the endpoints)?
Obviously everyone can measure the way they want. What is the value of measuring parallel to the ground?
This parallel to the ground business is not universal. Here is a page from Mark Bulgier's scans that suggests that some builders measure along the chainstay.
http://bulgier.net/
Of course this could just be an easy explanation for the slightly educated customer. As Don Ferris noted:
"Joe - When I talk about chainstay length to customers or regular bike folk, I'm talking about a straight line measurement from BB center to axle center (parallel). But when figuring chainstay length for rider balance on the bike or axle placement relative to the hanger eye, I'm talking about a true horizontal measurement, i.e., parallel to the ground. So yeah, the 6-9 would be a horizontal measurement, not a parallel one."
BTW, a brief look at some of the catalogs reveals that the rear axle is usually placed forward of the upper pivot bolt of the derailleur; in some cases, all the way forward.
Marcus Helman
Huntington Woods, MI