Originally Posted by biker7

You see many references today which are the result of a throw back to how frames have been measured for many years. With the onset of compact or sloped top tubed bikes...both seat tube and top bars now are typically rated in actual versus virtual which is the same as effective. What that means is...with a sloped top tube or compact geometry frame bike...actual seat tube length will be less than effective or virtual seat tube length for equivalent steerer tube length relative to bike size which historically has been from bottom bracket to top of top tube on conventional or horizontal top tube bikes. The way to measure the difference between actual and effective or virtual seat tube length for a compact/sloped top tube bike (there is no difference on a conventional horizontal top tube frame)...is take a carpenter's level and set is right on the top bar intersecting where the top tube connects to the steerer tube. The level will diverge from the top tube as it progresses rearward due to the slope of the top tube. Where it intersects above the seat tube or seat post defines the effective or virtual seat tube length "which gives the bike its size rating." Actual seat tube length is always less than this value for a compact frame bike.
HTH,
George