The successful realization of capabilities of variable resistors as elements of neuromorphic systems calls for a simple model that can reproduce the basic properties of such elements. The paper considers a model of a variable resistor using two parallel opposite simple resistor elements. The equations reducing the variation of a resistor's state to the movement of a representative point in the two-dimensional space are written and investigated. The model is shown to reproduce many properties of real variable resistors and allow us to evaluate their usability in particular data processing systems. Some potentially useful modes of operation of variable resistors are analyzed. The conditions under which a variable resistor can be used as a memory unit, sign-variable signal detector are found. The important role of characteristic functions for estimating variable resistors is pointed out.