The main difference between the Model 4640 Articulator and traditional Whip Mix articulators is the elimination of the crossbar between the uprights on the lower member. This frame design provides optimum posterior access for enhanced visibility and lingual access of any Whip Mix articulator. The 4000 series articulator also features the Accumount system which makes it possible to exchange mounted casts without loss of accuracy. There are two Models available in the 4000 series articulators, each with unique design features.