 One fundamental distinction in the realm of instant-based models of time is the distinction between linear models, where the flow of time is depicted as a line, and backward-linear models, which allow a tree-like representation, supporting the view that the past is fixed (and hence linear) while the future may be open (branching into multiple possible futures). In either case, the temporal ordering may or may not contain minimal or maximal elements, corresponding to first or last instants in time, respectively.