The idea that a gamemaster has the discretion to alter or discard published rules was not an invention of role-playing games: it derived from a wargaming tradition going back to the free Kriegsspiel of the nineteenth century. But role-players enshrined it as a principle that is today known as "Rule Zero", a proposed meta-rule of role-playing games -- albeit not an uncontroversial one. The critical position that we should hold this as a universal meta-rule occurred to the early adopters of role-playing games fairly early, as shown here, in the "Gamer's First Law" of Ed Simbalist (designer of Chivalry & Sorcery) in Alarums & Excursions #38 in 1978.
Famously, Dungeons & Dragons in 1974 described its system as a "framework around which you will build a game of simplicity or tremendous complexity." Its rules aspired to deliver merely a framework, not a set of iron dictums, because "as with any other set of miniatures rules they are guidelines to follow in designing your own fantastic-medieval campaign." D&D even suggests writing any new or changed rules into the rulebook in pencil, not pen, because "who knows when some flux of the cosmos will make things shift once again."
The use of the phrase "as with any other set of miniatures rules" signals that D&D's attitude towards the rules was not a novel one. Miniature wargames had long embraced referees who concealed secret information from players and ran the system in accordance with Kriegsspiel principles. D&D's language about guidelines most directly paraphrases Chainmail (1971), which stated, "These rules may be treated as guide lines around which you form a game that suits you." But this idea was certainly not exclusive to Gygax's work, it was commonly accepted that miniature wargame referees could adapt wargame rules to their liking. To take just three examples:
Modern War in Miniature (1966): "There is only one rule to our war game: simulate reality. The statistics and tables are designed to help the player in this task. When they get in the way, if they ever should, then you should discard them."
Grosstaktik (1972): "You should regard these rules as a bare framework. Apply them flexibly, and modify them freely in accordance with your own tastes."
Kam-Pain (1974): "The Gamesmaster is the final authority on all rules interpretations... he may freely alter or delete existing rules, and add new ones."
This principle was essential enough to Kam-pain that they codified it under the name of "The GM's Cloak." Kam-pain would, incidentally, fuel the "Midgard Ltd." fantasy campaign, one of D&D's closest cousins.
But codifying it into the rules is one thing, and proposing it as a law that transcends any individual set of rules is another: its transcendence could only become apparent with sufficient examples to draw on. This concept that the rules could be altered as the referee saw fit, once it appeared in D&D, would echo through the games that imitated and followed it, a growing tradition of fantasy role-playing games. Again, to take three examples:
Chivalry & Sorcery (1977): "Chivalry & Sorcery provides the guidelines by which players may easily create the kinds of worlds they want and does not attempt to 'dictate' in any way what must be."
Runequest (1978): "Take those portion of the rules you can use and ignore the rest. Like any FRP system, these can only be guidelines. Use them as you will."
Villains and Vigilantes (1979): "The rules presented in this book are made to be broken, as are all rules... If the Gamemaster feels that he disagrees with any part of these rules or any point made, he should, by all means, experiment and adapt the rules to suit his tastes and needs."
Given the stance of Chivalry & Sorcery, we should not be surprised to find Simbalist, in an offhand note in A&E #35, stating that "if a rule is silly, change it or ignore it." He received a reply from John Sapienza suggesting that that principle should be "engraved in bronze for all writers and designers to ponder." As this idea was per Runequest "like any FRP system," Simbalist was inspired to elevate it to the "Gamer's First Law," a meta-rule governing the operation of all role-playing game systems, "the foundation and mainstay of all FRPing."
However, flexibility was not the only possible approach to running FRP rules. Greg Costikyan contrasted these "open-ended" systems that permitted modification by the referee with "closed" systems with rules that were not intended to be modified. Early refereeless systems like En Garde (1975), gamebook modules like Buffalo Castle (1976), and of course computer role-playing games, showed how designers aimed for "closed" systems quite early on, and this too became a valid approach to FRP, one that has since existed in a tension with those who hold the "Gamer's First Law" as absolute.
So why is this maxim now remembered as "Rule Zero" and not the "Gamer's First Law" or "the GM's Cloak"? Internet RPG discussion forums inherited the idea of "Rule Zero" from earlier Usenet group discussions. My best guess is that Carl Henderson's discussion of FRP meta-rules beginning with a Rule Zero kicked this off, though his categories were quickly transformed by others, and his statement that "the gamemaster has the power to overrule any rule, precedent, combination of rules, or dice rolls" was originally his Rule Two. Ideas like this, once they enter the melting pot of the Internet, can be difficult to trace to their sources.
These Internet discussions, easily Googleable, effectively eclipsed any earlier theorizing in analog media like Alarums & Excursions. It is the ambition of The Elusive Shift to start the process of rectifying that.