Forty of the U.S. frequencies — eight in each zone — which came to be known as "Clear Channels", were generally limited nationally to a single station. The maximum power for these stations was to be determined later, and in most cases would be set at 50,000 watts. In some cases secondary stations were assigned the same frequency, with provisions to avoid interference to the primary station's coverage, by having the secondary stations located long distances from the primary station, using limited powers, or restricted to daytime-only operation.