In 1939, The National Broadcasting Company's (NBC) Red and Blue radio networks competed with the Columbia Broadcasting System (CBS) and the Mutual Broadcasting System in providing nationwide coverage. NBC advertising rate cards of the period listed "basic" and "supplemental" affiliated stations. Advertisers were encouraged to buy time for their programs on the full "basic" line-up (plus any "supplemental" stations they wished) but this was open to negotiation. It was not unusual for Red Network advertisers to place shows on Blue Network stations in certain markets (and the other way around). Supplemental stations were generally located in smaller cities away from the network trunk lines. Such stations were usually offered to advertisers on both the Red and Blue Network line-ups.