We say any order is a tolerance order on a set of vertices if we may assign to each vertex x an interval I_x of real numbers and a real number t_x called a tolerance in such a way that x y if and only if the overlap of I_x and I_y is less than the minimum of t_x and t_y and the center of I_x is less than the center of I_y. An order is a bitolerance order if and only if there are intervals I_x and real numbers t₁(x) and t_r(x) assigned to each vertex x in such a way that x y if and only if the overlap of I_x and I_y, is less than both t_r(x) and t₁(y) and the center of I_x is less than the center of I_y. A tolerance or bitolerance order is said to be bounded if no tolerance is larger than the length of the corresponding interval. A bounded tolerance graph or bitolerance graph (also known as a trapezoid graph) is the incomparability graph of a bounded tolerance order or bitolerance order. Such a graph or order is called proper if it has a representation using intervals no one of which is a proper subset of another, and it is called unit if it has a representation using only unit intervals. In a recent paper, Bogart, Fishburn, Isaak and Langley (1995) gave an example of proper tolerance graphs that are not unit tolerance graphs. In this paper we show that a bitolerance graph or order is proper if and only if it is unit. For contrast, we give a new view of the construction of Bogart et al. (1995) from an order theoretic point of view, showing how linear programming may be used to help construct proper but not unit tolerance orders.