Some educators caution that attempts to use d'Alembert inertial mechanics lead students to make frequent sign errors. A potential cause for these errors is the sign of the inertial forces. Inertial forces can be used to describe an apparent force in a non-inertial reference frame that has an acceleration a with respect to an inertial reference frame. In such a non-inertial reference frame, a mass that is at rest and has zero acceleration in an inertial reference system, because no forces are acting on it, will still have an acceleration − − a and an apparent inertial, or pseudo or fictitious force − − m a will seem to act on it: in this situation the inertial force has a minus sign.