George Lucas is more closely associated with his two hit franchises: Star Wars and Indiana Jones, two of the most successful adventure series in the entirety of film. Before he was reinventing the space opera genre or ‘30s serial film, however, Lucas was getting his start as an indie director on early projects like THX 1138 and American Graffiti – two equally exceptional films in their own right.

In the case of the former, THX 1138 served as the official feature-length debut for Lucas, an avant-garde, experimental sci-fi film that seemed like a spiritual successor to Metropolis and 1984. While the film was marginally well-reviewed at the time of its release, THX 1138 soon garnered renewed acclaim in the wake of Star Wars’ release, leading to it attaining cult status among larger audiences.

Though not nearly as well-loved as Lucas’s other series, the filmmaker has taken the time to insert passing references to his directorial debut into many of his films, subtly sprinkling them throughout Star Wars and Indiana Jones. For a helpful guide to each of these easter eggs, here is every reference George Lucas makes to THX 1138 in the Star Wars and Indiana Jones series.

Star Wars: A New Hope

Out of every Star Wars movie, A New Hope contains far and away the highest number of references to THX 1138. Leia’s cell block number on the Death Star is 2187, the futuristic date that the events of THX 1138 take place. As Luke, Han, and Chewbacca prepare to leave C-3PO and R2 in the Death Star control room, you can briefly see “THX/1138” pop up on the monitor behind 3PO.

When infiltrating the Death Star’s detention block under the guise of a stormtrooper, Luke tells the facility’s guards he is transferring Chewbacca to cell block 1138. In the original Star Wars script and its subsequent novelization, the Death Star stormtrooper whose suit is later used by Luke is referred to as THX 1138. In the finished film, he’s been renamed TK-421.

The Empire Strikes Back

With each new Star Wars film, the references Lucas made to THX 1138 became far more subtle and difficult to spot. In the case of The Empire Strikes Back, the movie’s single reference to THX 1138 comes when General Rieekan tells his subordinates on Hoth, “Send Rogues 10 and 11 to Station 3-8.”

Return of the Jedi

As with The Empire Strikes Back, the reference to THX 1138 in Return of the Jedi is a notably minor one and can be difficult for most fans to notice. Looking at Leia’s costume when she’s impersonating the bounty hunter Boussh on Tatooine, you might notice the number 1138 on the side of her helmet.

The Phantom Menace

As with the Original Trilogy, the number of references Lucas makes to THX 1138 in the Prequels are fairly low. In the trilogy’s first installment, the movie’s only reference can be spotted during the climactic battle between the Gungans and the Droid Army on the plains of Naboo near the end of The Phantom Menace. After Anakin destroys the Trade Federation’s command ship, the deactivated droid menacing Jar Jar very clearly has the number 1138 painted onto its back.

Attack of the Clones

In Attack of the Clones, an easter egg to THX 1138 is very briefly made during the Battle of Geonosis. Shortly after being rescued from the planet’s gladiatorial pits, Mace Windu orders a Clone gunship pilot to land at an assembly area near the battlefield’s front lines. The number on the back of the pilot’s helmet is actually 1138.

Revenge of the Sith

The reference to THX 1138 in Revenge of the Sith requires a more in-depth knowledge of Star Wars than the previous entries on this list. During the Order 66 sequence, Jedi Master Ki-Adi Mundi is ambushed by his clone troops on Mygeeto, and is subsequently gunned down by his former clone ally, Commander Bacara of the 21st Nova Corps. Bacara’s official designation in the Extended Universe is CC-1138.

The Force Awakens

Though George Lucas personally did not have any influence at all on the direction of the Star Wars Sequel Trilogy, the filmmakers continued the series’ tradition of referencing Lucas’s first film. For example, Finn’s serial designation is FN-2187. In terms of Star Wars trivia, this is a reference to Princess Leia’s cell block number on the Death Star in A New Hope, which in turn is a reference to the year of THX 1138’s dystopian setting, as mentioned previously. Additionally, a First Order stormtrooper who participates in the assault on Lor San Tekka’s village at the beginning of the film has the designation of 1138.

Rogue One

As far as we know, Rogue One doesn’t have any explicit nods to THX 1138 anywhere in the film. However, it’s worth noting that Darth Vader’s bacta tank (and therefore Darth Vader himself) is first introduced at 1:13:38. As interesting as this piece of trivia is, though, it’s highly unlikely the film’s editors added this scene in as a specific reference to THX 1138.

Indiana Jones: Raiders of the Lost Ark

Having gotten his major industry breakthrough with Star Wars, George Lucas and his collaborator Steven Spielberg added in quite a few homages to Lucas’s previous films when it came to film their inaugural Indiana Jones film, Raiders of the Lost Ark. Along with a humorous hieroglyphic sketch of R2 and 3PO near the Ark of the Covenant during the Well of Souls scene, a radio operator can be heard saying “Eins-eins-drei-acht” –  Eins-eins-drei-acht being the German translation for one-one-three-eight.

Indiana Jones and the Last Crusade

A common myth pervading Indiana Jones and the Last Crusade is that, during the Venetian church scene halfway through the film’s first act, careful observers might notice the Roman numerals “MCXXXVIII” written in Henry Jones’ grail diary. If you look at the journal very closely during this scene, however, you’ll notice that the numeral reference to THX 1138 simply isn’t there.

Indiana Jones and the Kingdom of the Crystal Skull

Another myth regarding Indiana Jones is that a license plate in the film’s opening moments bears the designation “THX 138.” As cool as that easter egg might’ve been, as with the previous reference in Indiana Jones and the Last Crusade, this reference is merely a myth, with the actual license plate reading “D5 594.” The only clear nod to Lucas’s debut film comes during the infamous “nuke town” sequence, with the suburban house Indiana Jones takes refuge in bearing the home address of “138.”