Together, they are sometimes referred to as the Big Three credit rating agencies. While credit rating agencies are sometimes viewed as interchangeable, Moody's, S&P and Fitch in fact rate bonds differently; for example, S&P and Fitch Ratings measure the probability that a security will default, while Moody's ratings seek to measure the expected losses in the event of a default. All three operate worldwide, maintaining offices on six continents, and rating tens of trillions of dollars in securities. However, only Moody's Corporation is a free-standing company.