Leroy Jethro Gibbs' rules appear in the series "NCIS"; Gibbs and his team cite the rules. Rule number one is "Never let suspects sit together." Rule number three is "Never believe what you are told. Double check." A fan-compiled list of Gibbs' rules is available at NCIS.wikia.com.
During the series, rules number one and three have been duplicated. The second version of rule number one is "Never screw over your partner." The second version of rule number three is "Never be unreachable."
According to executive producer Shane Brennan, this duplication was intentional. One version of rules one to three belongs to Gibbs; the other was adopted from his first partner Mike Franks. As of season 12, only one version of rule number two has been revealed: "Always wear gloves at a crime scene."