The original chiming sound was created by Apple programmer Jim Reekes, who designed many of the system sounds for the Mac OS, including that iconic startup chime roughly 20 years ago. Apple has used several different variations on the chime over the years, including a shift in pitch to the current tone Apple notes as having been in use since early 1999.
On the trademark application, which was filed in June of this year, Apple states that the chime is "a sound mark consisting of a slightly flat (by approximately 30 cents) G flat/F sharp major chord."
Apple uses the sound to signal that the diagnostic tests run at the startup of a Mac computer have found no hardware or software problems.
Other Apple trademarks include slogans like "There’s an app for that" and "Think Different," and product labels like "Retina," "Time Capsule," and "Smart Cover."