Rotational discontinuities are associated with the reconnection configuration on the dayside magnetopause and the energy transfer process in the tail magnetopause. In the present paper we develop a model of rotational discontinuity in which the ion dynamics is described by the fluid equations and electrons are assumed to move adiabatically along the magnetic field lines. This model is valid for a rotational discontinuity with a thickness greater than a a few ion gyroradii. By including trapped electrons self-consistently in our model, we show that (1) the trapped electron density profile is uniquely related to the rate of angular rotation of the magnetic field and (2) the sense of magnetic field rotation is determined by the ratio of the ion thermal and ion streaming energies and by the ratio of the normal and tangential components of the magnetic field. It is found that magnetopause rotational discontinuities should have the electron polarization in agreement with satellite observations.