Mode coupling refers to polarization state changes of the x - and o -modes in response to different plasma conditions. If a radio wave passes through a region where the magnetic field orientation is nearly perpendicular to the direction of travel, which is called a quasi-transverse region, the polarization sign (i.e. left or right; positive or negative) may flip depending on the radio frequency and plasma parameters. This concept is crucial to interpreting polarization observations of solar microwave radiation and may also be important for certain low-frequency radio bursts.