The Big Bang Theory 20th Anniversary 2007-2027 Bazinga Commemorative Coin

Knock, knock, knock. “Penny.” Knock, knock, knock. “Penny.” Knock, knock, knock. “Penny.” Anyone who watched The Big Bang Theory can hear that rhythm without trying, the ritual Sheldon Cooper performed at every door he needed opened. It runs across the top of the back of this coin as “Penny! Penny! Penny!” with the knocks tucked underneath in parentheses, and it is the quickest way to tell a casual viewer from a devoted one.

The show premiered on CBS on September 24, 2007, and signed off on May 16, 2019, after twelve seasons and 279 episodes. Chuck Lorre and Bill Prady built a formula sturdy enough to keep growing after the finale: Young Sheldon ran from 2017 to 2024, Georgie & Mandy’s First Marriage followed in 2024, and Stuart Fails to Save the Universe arrived on HBO Max on July 23, 2026. Counting from that first episode, 2027 marks twenty years.

The front, with the gang, the couch, the whiteboard and a Bazinga! with a 73 beside it.

Every doodle on the coin points back to an episode

The design rewards people who have seen the show more than once. Here is how the two faces break down.

Detail	Face	Where it comes from
“Penny! Penny! Penny!” and three knocks	Back	Sheldon’s door ritual, three knocks and a name, repeated three times
The number 73	Front	Sheldon’s favorite number, explained in “The Alien Parasite Hypothesis,” season 4, episode 10
The couch and “That’s my spot”	Front	The one cushion in apartment 4A that nobody else was allowed to claim
The astronaut	Front	Howard’s trip to the International Space Station at the end of season 5
The cat	Front	“Soft Kitty,” the song Sheldon insists on hearing whenever he is sick
The atom in place of a zero	Back	The physics at the heart of the show
“Bazinga!”	Both	Sheldon’s way of announcing he was joking

The back, with three knocks, three Pennys and an atom standing in for the zero.

Seventy-three turned out to be real mathematics

The 73 is the best joke on the coin because it is not really a joke. In “The Alien Parasite Hypothesis,” which happens to be the show’s 73rd episode, Sheldon explains that 73 is the 21st prime number, that its mirror 37 is the 12th prime, that 12 mirrored is 21, which is 7 times 3, and that in binary 73 reads 1001001, the same backward and forward.

It sounded like pure Sheldon, the kind of speech that leaves everyone else at the table staring into their takeout. Then in 2019, mathematicians Carl Pomerance and Chris Spicer published a proof that 73 is the only number with those properties, and they named it the Sheldon prime. Very few sitcom lines end up in a math paper, and fewer still get printed next to a cartoon couch.

A small coin for a show that keeps getting rewatched

Twenty years is long enough for a sitcom to turn into comfort viewing, and this one did. Plenty of people who watched the premiere now share the reruns with family who first met Sheldon through Young Sheldon.

It makes an easy gift for the friend who says Bazinga at work, the scientist with a whiteboard at home, or the roommate who absolutely has a spot on the couch. Sheldon’s lifelong devotion to Star Trek also makes it a fitting neighbor for our Star Trek 60th anniversary coin in a display case.

Front and back side by side, against a blackboard full of equations.

Flip it to settle an argument or tuck it into a card for someone who still quotes the show daily. Just knock three times before you hand it over.

Written by Nadia Marsh, Collectibles Specialist at DoesignUs.