If you don’t pop in your noise-canceling headphones the moment you take your seat on a flight, you’ll eventually hear a familiar sound: a “ping,” used by the crew to communicate with passengers. That non-threatening sound signals when you should stay seated and when you’re free to get up, for example. But sometimes those sounds aren’t meant for the customers. As People reports, airline crews use the chimes as a Morse code of sorts to alert each other to issues big and small.

Qantas Airways shared the code used on their flights in a blog post last year. The high-low “ding-dong” chime that can be heard ringing throughout the cabin is the most common way staff in different parts of the plane get each other’s attention. According to Qantas, this sound is the “ringtone of a crew phone from one galley or section to another.” These calls are usually made for non-pressing matters, like checking to see what the pretzel situation is like in another galley when one area runs out.

There’s also the triple low chime, which is reserved for priority messages from the captain or crew. This can be used to convey possible turbulence, in which case the flight attendants have time to secure their snack carts before an official announcement is made.

While this system is standard across Qantas Airways, it varies from airline to airline. Retired U.S. Airways captain John Cox shared his own insider information in a blog post for USA Today. In his experience, two chimes indicate the plane is approaching 10,000 feet, while three or more chimes signal either turbulence or a sick passenger in need of medical attention. One chime can also communicate bumpy skies, but it’s more commonly used by plane captains to ask for a coffee refill.

Check out more behind-the-scenes secrets from flight attendants here.

[h/t People]