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Copied: In general, the reason for which a stock is traded over-the-counter is usually because the company is small, making it unable to meet exchange listing requirements. Also known as "unlisted stock", these securities are traded by broker-dealers who negotiate directly with one another over computer networks and by phone.
Although Nasdaq operates as a dealer network, Nasdaq stocks are generally not classified as OTC because the Nasdaq is considered a stock exchange. As such, OTC stocks are generally unlisted stocks which trade on the Over the Counter Bulletin Board (OTCBB) or on the pink sheets