Gygax, who wrote the advanced game, wanted an expansive game with rulings on any conceivable situation which might come up during play. J. Eric Holmes, the editor of the basic game, preferred a lighter tone with more room for personal improvisation. As a result, the basic game included many rules and concepts which contradicted comparable ones in the advanced game. Confusing matters further, the original D&D boxed set remained in publication until 1979, since it remained a healthy seller for TSR.
Advanced Dungeons & Dragons was designed to create a tighter, more structured game system than the loose framework of the original game. While seen by many as a revision of D&D, AD&D was at the time declared to be "neither an expansion nor a revision of the old game, it is a new game". The AD&D game was not intended to be directly compatible with D&D and it required some conversion to play between the rule sets.
In 1981 Basic Dungeons & Dragons was revised by Tom Moldvay. However, the rules for the Dungeons & Dragons game continued to diverge and it became a separate and distinct product from TSR’s flagship game, AD&D. This game was promoted as a continuation of the original D&D tone, whereas AD&D was an advancement of the mechanics. Although simpler overall than the 'Advanced' game, it included rules for some situations not covered in AD&D.