On The Big Bang Theory

As an adult, Sheldon Cooper has a ritualistic knock that he uses whenever he's knocking on somebody's door. As established on The Big Bang Theory, Sheldon knocks three times and says the person's name, and he performs this three times.

Sheldon's knock is most associated with him going across the hall to Penny's apartment.

On Young Sheldon

Sheldon has not yet fully developed his later knocking pattern. In its embryonic form, Sheldon still knocks three times and calls the person's name, but he doesn't yet do this in triplicate. So far, Sheldon's knock is most associated with him going across the street to Meemaw's house.

Sheldon's knock was first seen in the episode "A Patch, a Modem, and a Zantac", when Sheldon visited Meemaw's house to ask for money to buy a computer. He knocked three times and said Meemaw, but he only did this once.

The knocking ritual was extended to two rounds in "An Eagle Feather, a String Bean, and an Eskimo", as Sheldon visited Meemaw to show off the uniform for his new school.

Sheldon stuck with two rounds when he visited Meemaw in "Summer Sausage, a Pocket Poncho, and Tony Danza" to ask her about her date with Dr. John Sturgis. When Sheldon woke Georgie up for help with his paper route in "A High-Pitched Buzz and Training Wheels", he performed two rounds of his knock on Georgie's headboard.