Once Dungeons & Dragons was published and was really taking off, it became clear that the game—the booklets themselves—wasn’t very accessible. I’m sure whilst writing them, Gary never dreamed the game would end up on the shelves of a mall bookstore. He was writing to fellow wargamers. Along came Dr. John Eric Holmes with a proposal to do remedy that. Thus, the first Dungeons & Dragons Basic Set.
Dr. Holmes brought some of his own ideas about how the game should work. Later, as the successor to his Basic Set was coming out, he wrote an article for Dragon magazine in which he discussed some of the changes to the game he wanted to make that Gary veto’d. Also, TSR decided that first Basic Set should include bits of and advertising for the forthcoming Advanced D&D game.
I never had the Holmes Basic Set—though I bought the PDF and have it around somewhere—so I don’t know it very well. Jeff posted the following quote from it (emphasis mine):
A final word to the Dungeon Master from the authors. These rules are intended as guidelines. No two Dungeon Masters run their dungeons quite the same way, as anyone who has learned the game with one group and then transferred to another can easily attest. You are sure to encounter situations not covered by these rules. Improvise. Agree on a probability that an event will occur and convert it into a die roll — roll the number and see what happens! The game is intended to be fun and the rules modified if the players desire. Do not hesitate to invent, create and experiment with new ideas. Imagination is the key to a good game. Enjoy!
That pretty much lays out the “generic unified unsystem” that I try to use with any system. I think it is a subtext in many early role-playing games. (And now I know it’s a part of the text of at least one as well.) I particularly like that he used the word “agree”.