NCIS fans know that Gibbs has a list of tenets he lives by, but Tuesday's season finale, “Rule Fifty-One,” is perhaps the most overt reference to Jethro's unwritten code.

Naturally, the definition of the 51st rule is being closely guarded, but executive producer Shane Brennan did offer this: "Fans are about to learn three rules in one episode.”

“At the end, they’ll see Gibbs compose one. He makes a difficult choice, and sees something so unexpected he hasn’t got a rule for it, so he articulates one and adds it to the list. It’s a very, very poignant moment, pertinent to the rules, that will tug at the heartstrings.”

The known rules of Gibbs range from memorable to forgettable, and from longstanding to only recently unveiled (such as Rule #40 in this month's "Borderland").

While it's believed Gibbs has 50 rules - which he once told Ziva, and the even 50 may account for the fact that he's improvising a 51st - there are plenty we haven't heard.

Brennan says Rule #51 is a climax to the season finale, but not the climax.

“If they think that’s the surprise, it’s not. Anyone who watches the show gets it. They know they have to watch the last 30 seconds of the last episode of the season.”

The cliffhanger is "not violent. But the anticipation of violence, grief, and hurt, it’s astonishing. I mean, the audience will go, 'Oh my God, they’re not going to do that.' The last 20, maybe 30 seconds of Season 7 will have people talking all summer. It’s very personal."

Also unclear, on a side note: Rule #44. Gibbs urgently tried to get this message to Mike Franks in "Patriot Down" and while its context was clear, it was never explained.

Theories on what Rules #44 and #51 are, or what the season finale will entail? Sound off below - and follow the jump for a great list TV Guide compiled of Gibbs rules ...

Rule #1: Never let suspects sit together. (Yankee White: episode 1)
Rule #1: Never screw over your partner. (Blowback: episode 84)
Rule #2: Always wear gloves at a crime scene. (Yankee White: episode 1)
Rule #3: Never believe what you are told. (Yankee White: episode 1)
Rule #3: Never be unreachable. (Deception: episode 58)
Rule #4: Best way to keep a secret is to keep it to yourself. Second best way is to tell one other person if you must. There is no third best. (Blowback: episode 84)
Rule #6: Never say you’re sorry. (Flesh and Blood: episode 150)
Rule #7: Always be specific when you lie. (Reveille: episode 23)
Rule #8: Never take anything for granted. (Probie: episode 56)
Rule #9: Never go anywhere without a knife. (One Shot, One Kill: episode 13; Missing: episode 20; Probie: episode 56)
Rule #10: Never get involved personally on a case. (Obsession: episode 159)
Rule #11: Walk away when the job’s done. (Semper Fidelis: episode 137)
Rule #12: Never date a co-worker. (Enigma: episode 15)
Rule #13: Never involve lawyers. (Collateral Damage: episode 119)
Rule #15: Always work as a team. (Leap of Faith: episode 99)
Rule #18: It's better to seek forgiveness than ask permission. (Silver War: episode 51)
Rule #22: Never, ever bother Gibbs in Interrogation. (Smoked: episode 80)
Rule #23: Never mess with a Marine's coffee if you want to live. (Forced Entry: episode 33)
Rule #27: There are two ways to follow someone. First way, they never notice you. Second way, they only notice you. (Jack Knife: episode 153)
Rule #38: Your case, your lead. (Bounce: episode 129)
Rule #39: There is no such thing as coincidence. (Obsession: episode 159)
Rule #40: If it seems like someone is out to get you, they are. (Borderland: episode 160)

You're probably asking: Why are there two No. 1s, and two No. 3s? Aficionados have wondered if, worst case scenario, those were continuity errors, or, giving the NCIS writers the benefit of the doubt, if Gibbs’ list was simply meant to be malleable.

The answer is: neither. Here is Brennan’s surprising explanation:

"Gibbs lives his life by a set of rules that took root from the first day he met Shannon. Over time, Gibbs added to the rules. When he joined NCIS, Mike Franks told him he didn't need dozens of different rules to be an agent... just three 'golden rules.'"

"This is why we have double ups on rules #1 and #3. Three of them are Gibbs' rules; three of them are Mike Franks’ rules. We are still to reveal the double-up on rule #2. It's up to fans to guess which of the rules were Mike Franks’ and which were Gibbs'."

Steve Marsi is the Managing Editor of TV Fanatic. Follow him on Google+ or email him here.