Those are just the sweet tones of your airplane crew speaking their own coded language.

Many airplane travelers are well aware of the particular “ding” sound that rings through the cabin while they’re on a flight. Most of the time, it could mean the seatbelt sign has been turned off or on, but the familiar tones are also part of an airplane crew shorthand.

In a blog post by Qantas Airways, the airline explained the different types of chimes you might hear during your trip. As it turns out, there isn’t just one person constantly calling the flight attendant or a practical joker co-pilot trying to ring you out of your slumber.

“There can be up to 30 crew both cabin and pilots spread throughout an aircraft on a long haul flight. So, good communications are crucial. That’s where all the bells come in,” the airline states. “On our Airbus aircraft you’ll hear the ‘boing’ sound shortly after take-off – this sound lets crew know that the landing gear is being retracted…The second boing is usually when the seat belt sign is switched off.”

The various chimes and their frequency matter, too. Apparently, the difference between a single and a triple “ding” depends on the urgency of the situation.

Qantas uses a single chime for passengers needing service and a “high-low” ringtone for crew members to call other crew members in other parts of the cabin (for non-emergency things).

Three low chimes in a row means there is turbulence ahead and all crew should put away any meal carts and fasten their seatbelts.

Of course, each airline uses a customized system. According to the Huffington Post, United Airlines not only uses a single chime for passengers calling for service, but also to note that the seatbelt sign has been turned on or off.

And a spokesperson for Southwest Airlines told HuffPost that the airline uses a combination of tones communication in the cabin, but “declined to specify what each sound means, citing security.”

This article originally appeared on TravelandLeisure.com