Refraction and/or magnetic field curvature can lead to 'geometrical mode coupling' between the MHD modes. The effect is linear, but it is a finite-wavelength effect. It is studied here for a simple configuration which is amenable to analysis and which illustrates the basic features of the coupling. In the solar wind the geometrical coupling may be operative only in the solar corona or in the interaction regions of high-speed streams. In the later case the geometrical coupling may provide an explanation for the non-Alfv¿nic fluctuations but only for long-period waves (greater than several hours) in small interaction regions (<0.1AU).