Trek's Kammtail Virtual Foil (KVF) is an aerodynamic tube shape that maintains its full drag-reducing benefits at every yaw angle (the direction of the wind on your frame) by removing the trailing edge of the airfoil (compare the top and bottom profiles in the image above). The result? Lightweight, aerodynamic tubing that makes your bike faster in every situation, from headwinds to tailwinds and everything in between.
For increased stiffness and improved steering precision, tapered head tubes (see image, on left) and forks pair an oversize lower steerer tube (usually 1 1/2-inch, see image, on right) with a standard diameter upper steerer tube (usually 1 1/8-inch). This combination sheds weight compared to straight 1 1/2-inch systems, and adds stability and stiffness when compared to straight 1 1/8-inch systems. The result is a bike that tracks better through corners while allowing for the use of standard stems.
This manufacturer's chart helps you check the fit (may show unavailable, and/or not all sizes).