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, the company's investment banks and NYSE's floor traders to get a stock trading.
Every NYSE-listed has a DMM. DMMs, previously known as specialists, are unique to the NYSE.
If technical problems arise, the NYSE uses DMMs to bypass electronic trading systems, allowing humans to trade a company's stock. That is not possible on all-electronic stock exchanges such as the Nasdaq.
The Nasdaq's trading system had technical problems during Facebook's IPO last year, which led to trading delays and problems with orders.
Barclays' role as Twitter's DMM does not mean it is in charge of the entire IPO process. That role falls to Twitter's investment banks: Goldman Sachs, Morgan Stanley and JPMorgan Chase.
Barclays is the NYSE's second-biggest market-making firm, representing more than 700 companies on the floor of the exchange.
The British company acquired the business when it bought parts of Lehman Brothers after that firm's bankruptcy in Sept. 2008. Barclays has since added other firms to its DMM business.