  Vector fields and line fields, their counterparts without orientations on
tangent lines, are familiar objects in the theory of dynamical systems. Among
the techniques used in their study, the Morse--Smale decomposition of a
(generic) field plays a fundamental role, relating the geometric structure of
phase space to a combinatorial object consisting of critical points and
separatrices. Such concepts led Forman to a satisfactory theory of discrete
vector fields, in close analogy to the continuous case. In this paper, we
introduce discrete line fields. Again, our definition is rich enough to provide
the counterparts of the basic results in the theory of continuous line fields:
a Euler-Poincar\'e formula, a Morse--Smale decomposition and a topologically
consistent cancellation of critical elements, which allows for topological
simplification of the original discrete line field.
