 The most significant such taboo restricts the allocation of stations 10.6 and 10.8 MHz apart, to protect against mixing products which will interfere with an FM receiver's standard 10.7 MHz intermediate frequency stage.Commercial broadcasting is licensed only on channels 221 through 300 (the upper 80 channels, frequencies between 92.1 and 107.9 MHz), with 200 through 220 (the lower 21 channels, frequencies between 87.9 and 91.9 MHz) reserved for non-commercial educational (NCE) broadcasting.In some "Twin city" markets close to the Canada–United States or Mexico–United States border, such as Detroit, Michigan, and Windsor, Ontario, or San Diego, California, and Tijuana, Baja California, commercial stations operating from those countries target U.S. audiences on "reserved band" channels, as neither Canada nor Mexico has such a reservation.