The Octalink system uses a spindle with eight splines. The splines provide a contact area between crank and spindle for an interface. Octalink exists in the marketplace in two variants, Octalink v1, and Octalink v2. The difference between the two can be seen by the depth of mounting grooves on the bottom bracket spindle. V1 spline grooves are 5 mm long, while V2 grooves are 9mm long. Shimano 105, Ultegra 6500 and Dura Ace 7700 cranksets mate to version one bottom brackets, while later mountain bike designs use the deeper-grooved version two. The system is proprietary and protected by Shimano patents and license fees.