In Season 20, NCIS had to let go of one of the CBS show's best recurring elements: Gibbs' rules. Mark Harmon's character Leroy Jethro Gibbs wasn't just a fan-favorite, but he also kept the team in line through a lengthy list of rules. While the series may not be struggling in his absence, his replacement Alden Parker doesn't have a set of rules to guide NCIS by.

The 20th season has involved a lot of personal storylines, including Nick Torres possibly being killed off as he deals with personal drama. Other episodes have included Parker's ex-wife, Jessica Knight's sister and Timothy McGee's wife. The family-centric focus is clearly an effort to keep the tone that NCIS maintained in the Gibbs years. His rules likewise left a distinct impression on his team, both in the past and the present.

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NCIS Gibbs' Rule #12: Never Date a Coworker

NCIS' Tony DiNozzo and Ziva David in the NCIS office, looking off camera

The Gibbs rule that almost every NCIS fan knows is Rule #12: "Never date a coworker." Originally, it was a passing reference in Season 1, Episode 15, "Enigma" and the reason that former NCIS agent Kate Todd would never be able to date Tony DiNozzo. However, the rule took on a life of its own and became a running gag -- because everyone started to break Gibbs' edict. From DiNozzo ending up with Ziva David to Lucy Tara and Kate Whistler's relationship on NCIS: Hawai'i, it's like the rule doesn't exist anymore. That's just as well since Gibbs gave Jack Sloane a heartfelt kiss when she left the team in Season 18.

NCIS Gibbs' Rule #5: You Don’t Waste Good

NCIS DiNozzo Baltimore Cop

Another meaningful Gibbs rule was Rule #5: "You don't waste good." It was first mentioned in Season 8, Episode 22, "Baltimore," which flashed back to DiNozzo's time as a cop. When Gibbs worked a case with DiNozzo, they found out that the police chief was corrupt. DiNozzo then decided to quite the force -- and Gibbs offered him a job at NCIS.. He could see DiNozzo's potential and wanted to capitalize on it, and the two made a great team until actor Michael Weatherly departed the show in Season 13.

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NCIS Gibbs' Rule #10: Never Get Involved Personally on a Case

NCIS Gibbs

One of Gibbs' most storied rules was Rule #10: "Never get involved personally on a case." It was introduced in Season 7, Episode 21, "Obsession," which involved the KGB and hidden money. Along the way, DiNozzo fell in love with a woman who was targeted with an incurable poison. After she died, DiNozzo admitted to breaking the rule and Gibbs told him that it was a rule he regularly struggled with as well.

In Season 16, Gibbs had a change of heart on Rule #10. Ellie Bishop found one of Ziva's old notebooks she became heavily invested in an old case. Gibbs got mad and threw her off the investigation, but a few episodes later, Gibbs told McGee that he had burned Rule #10. He had finally realized that getting personally involved was what made his team so good at their jobs. For that reason, getting rid of Rule #10 might be the most important thing Gibbs ever did.

NCIS Gibbs' Rule #91: When You Decide To Walk Away, Never Look Back

NCIS Gibbs' Boat Header

Gibbs last rule was Rule # 91: "When you decide to walk away, never look back." It first appeared in Season 18, Episode 16, which was actually called "Rule 91." That was Bishop's last episode so it detailed her exit from NCIS. Unfortunately, NCIS fans would learn that Rule #91 would also apply to Gibbs the very next season. In Season 19, he decided to stay in Alaska -- embracing Rule #91 and moving on to the next phase of his life.

NCIS airs Monday nights at 9:00 p.m. on CBS.