The seat tube length (BB to top of top tube) measurement is available in two forms. The first dimension measures the distance from the center of the bottom bracket along the centerline of the seat tube to the point where a horizontal line would intersect the top edge of the top tube where it meets the seat tube. The second dimension measures the distance from the center of the bottom bracket along the centerline of the seat tube to the point where a horizontal line would intersect the top edge of the top tube where it meets the head tube. These two dimensions are illustrated above.