The general rule is that a slacker head angle requires a fork with more offset, and small wheels require less offset than large wheels. The length of the fork is usually measured parallel to the steerer tube from the bottom of the lower bearing race to the center of the front wheel axle. A 1996 survey of 13 700c road forks found a maximum length of 374.7 mm and a minimum of 363.5 mm. The width of the fork, also called spacing, is measured colinear with the front wheel axle between the inside edges of the two fork ends. Most modern adult sized forks have 100 mm spacing. Downhill mountain bike forks designed for through axles have 110 mm spacing. The steerer tube is sized either to just accommodate the headset bearings, in the case of a threaded headset, or to contribute to the desired handlebar height, in the case of a threadless headset. When sizing a fork to a frame, the diameter of the fork steerer or steer tube (1" or 1⅛" or 1½") must not be larger than that of the frame, and the length of the steerer tube should be greater than but approximately equal to the head tube length plus the stack height of the headset. Adapter kits are available to enable use of a 1" fork in a frame designed for a 1⅛" steerer tube or a 1⅛" fork in a 1½" frame. Manufacturers of high-end bikes, both road and mountain, have started to use tapered steerer tubes. While there are purported advantages, there are not any standards yet developed, with each manufacturer following its own conventions. This makes replacement parts difficult to come by, only available from the original manufacturer. The blades must be the proper length to both accommodate the desired wheel and have the correct amount of rake to provide the approximate steering geometry intended by the frame designer. The functional length of the fork is typically expressed in terms of Axle-to-Crown race length (A-C). Also, the axle on the wheel must fit in the fork ends (usually either a 9mm solid or hollow axle, or a 20mm thru-axle). Some manufacturers have introduced forks and matching hubs with proprietary standards, such as Maverick's 24mm axle, Specialized 25mm thru-axle and Cannondale's Lefty system. Fork steerer tubes may be threaded or unthreaded, depending on the headset used to attach the fork to the rest of the bicycle frame.