Robert Greifeld, CEO at Nasdaq OMX Group, said the launch of the Price Size Exchange represents the "most fundamental change in market structure" since the opening of the Nasdaq Stock Market in the early 1970s. Unlike other exchanges, the PSX puts the size of an order first, before the time that an order arrives. "The basic structure of U.S. markets has held as its core principle price time," Greifeld said. "So if you were faster to the market, you were able to get the execution." At PSX, however, "the core principle is price size."