They are made possible by the release of later editions' rules in a System Reference Document under the terms of the Open Game License, which allow the use of much of the proprietary terminology of D&D that might otherwise collectively constitute copyright infringement.
Castles & Crusades is one example, using the unified d20 mechanic from 3rd edition while dropping what the developers perceived as complications (including feats, skills, and prestige classes).
[citation needed] Role-playing game publisher Matthew Finch was involved in the development of Castles & Crusades, serving as editor of the Player's Handbook, and was the initial author of OSRIC, which was afterward taken up by Stuart Marshall and released to the public in 2006 as a retro-clone of the first edition of Advanced Dungeons & Dragons (1977–1989).
The following year, Finch announced the release of Swords & Wizardry, a retro-clone of the original Dungeons & Dragons game.
OSRIC, short for Old School Reference and Index Compilation, is a recreation of the first edition of Advanced Dungeons & Dragons and one of the most successful retro-clones.
[2] Labyrinth Lord (LL) is a retro-clone written and edited by Daniel Proctor and published by Goblinoid Games in 2007.
Mazes & Perils RPG (M&P) is a retro-clone developed by Wild Games Productions, emulating the 1977 version of the Basic Set edited by John Eric Holmes.
The intended features of this retro-clone are 100% faithfulness and compatibility with the original rules it emulates; brevity; clarity; and meticulous organization and indexing.
Old School Essentials is unique in that both versions come in two different formats: All-in-one (or two) rules tomes or modular multi-book sets.