Empty channel spaces in the frequency allocation table to which broadcast channels cannot be assigned due to potential interference. The most obvious one is the co-channel taboo: two different television or radio stations cannot operate on the same frequency in the same geographical area. Other taboos cover geographical spacing for adjacent channels and for “images” (spurious frequencies akin to aliases) that are caused by reception in existing television sets. The taboos effectively knock out much of the UHF television band, so some ATV proponents wonder whether they might be too strict.