Here’s luggage advice all true travel pros should know.

Once you’ve navigated the nightmare of airport security, your next challenge is fitting your suitcase into the overhead bin on the plane — and it’s not as straightforward as it seems.

According to Daniel Compton, a flight attendant with Delta, the size and shape of overhead bins can vary by aircraft model. He shared with Travel + Leisure that doing some homework on your specific flight’s plane can make the stowing process much smoother.

“Our fleet at Delta consists of a variety of aircraft, which means that each has unique overhead bins. Thus, there’s no single correct way to fit your luggage, as the bins differ in their dimensions,” Compton explained.

For example, overhead bins in Delta’s A321 CEO fleet should have suitcases stowed lying down, but the A321 NEO fleet will have luggage stored on its side, he explained.

“You can always ask a Delta flight attendant which way is best for the plane you’re flying on. There are also often announcements during boarding or placards with information on many of our aircraft.”

A general rule of thumb to follow would be to store your carry-on bag wheels-first and vertically, like a book on a shelf, Denise Margrét Yaghi, a flight attendant with Play Airlines, shared.

“This saves space and makes it easier for others to store their bags as well. If it doesn’t fit that way, lay it wheels-first horizontally, but always be mindful of space and weight,” she said.

However, not all carry-on items should be placed in the overhead bin.

“Be sure you’re only placing items that fit properly in the overhead bins, and ensure items are properly closed,” Compton said. “You don’t want anything spilling out on another customer.”

If you’re always carrying small items, such as a purse, it should be put under the seat in front of you. Meanwhile, heavier items should also be stowed under a seat, as it could be difficult and dangerous to store overhead, where it could be unwieldy to retrieve.

Former flight attendant Kristine Eksteine Nizka added that anything you might need during the flight should not be put in a bag being stored in the overhead bin.

“So, don’t leave your entertainment, medicine, snacks, and water in the overhead locker,” she said. “It’s not only about bothering other passengers, but if the seatbelt sign is on for some reason, you will not be allowed to stand up and get those things.”

If you can’t find space, it’s OK to ask for help from a flight attendant — but they won’t put your bag in the overhead bin themselves.

“You can always ask a flight attendant for tips on how to best stow your suitcase,” Compton said. “We are always happy to answer questions from our customers. We do ask that you handle your own suitcase unless you have a specific condition or reason you are unable.”

Flight attendants aren’t allowed to lift bags due to safety reasons. Flight attendants don’t get paid until all passengers are on board and the plane doors are shut, so if they get hurt, they don’t have workers’ compensation because they’re not on the clock yet.

Of course, it would be ideal to store your carry-on above your own seat, but it’s possible that it may have to go above in a different area of the plane since the overhead bins are a shared space.

“Don’t take up more space than needed, and always close the bin when you’re done,” Yaghi said. “And if you’re unsure about anything, we’re always here to help make your flight smooth and safe.”