NEW YORK (AP) -- When Twitter goes public on the floor of the New York Stock Exchange, it will trade under the supervision of Barclays Capital.

Barclays Capital says the social media company has hired it as a "designated market maker," a critical role when a stock first starts trading on the NYSE. Twitter's stock is expected to debut on Thursday.

A DMM supervises the trading of a company's stock. He or she is an experienced trader in charge of ensuring that buying and selling go smoothly. If trading becomes volatile, the DMM can step in and buy shares using his or her firm's own money.

DMMs are especially important the day a company goes public, because the DMM coordinates between Twitter, its investment banks and the NYSE's floor traders to get a stock trading.

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