Professional piercer Courtney Morrison told Byrdie that "a nasallang piercing is a single bar placed through the right nostril, the septum and the left nostril" — sort of like a bridge piercing but further down. It looks like two separate piercings, one in each nostril, but the bar is just a single piece of model going from one side of your nose across to the other. They cross through the septum — and are for this reason sometimes also called a tri-nasal piercing — so in one sitting you're getting both nostrils and your septum pierced. 

Surely that's gotta be pretty painful, right? Yup, you'll need a decent pain tolerance for this piercing, and it can be a bit more expensive than simpler piercings as you'll probably need a fairly experienced piercer too. Byrdie describes the pain as being "somewhere in the 8 out of 10 range," so it's not the most painful piercing out there, but it will definitely hurt. You'll have to look after it well to avoid infection, too.