Traditionally, the bike industry determined the size of a frame by measuring along the seat tube from the center of the bottom bracket to the top. A frame from that era had a horizontal top tube so the seat tube ended at the same height as the upper head tube.
On a modern bike frame, often the top tube slopes, so the top of the seat tube is lower than the top of the head tube. This sloped design provides more standover height, as well as other benefits.
With a modern, sloping top-tube design, the "actual" seat tube measurement is shorter for a given head tube height. But how much shorter is determined by the frame design without any correlation to the size of rider. Despite this, the industry continues to refer to seat-tube sizing, so we use the "virtual" measurement (based as before on head tube height) to allow you to accurately compare sizes. In other words, if you draw a horizontal line from the top of the head tube to the “invisible” seat tube, and measure the full “invisible” seat tube; you can determine the virtual frame sizing.
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