Expert Set (BX D&D)
From D&D Wiki
The first official basic set was soon followed by David "Zeb" Cook writing an expansion, titled the "Expert" rules for characters level 4-14. This began a tradition. When the basic line was revised, these two books came to be distinguished as B/X D&D. This expansion was created because players found it difficult to transition between BD&D and AD&D at early level, and the compiled rulebooks (Both the basic rules and basic set) gave no clear guidance on how to progress any farther. It was believed that by providing higher levels at which a transition could be made from one edition to another would ease the conversion of the community to the new edition. However, instead of having the anticipated effect of converting old gamers to a new system, it created a new faction of D&D players- those who just loved the style of game presented in the basic rules sets.