Key
Squads
Characters
The Codex Astartes
The Codex Astartes was created by the Ultramarine Primarch Roboute Guilliman and others in the wake of the Horus Heresy. It was designed to prevent the conditions that led up to the conflict, and protect the Space Marines from further corruption by Chaos.

New guidelines were set regarding the creation of new Space Marines, involving slower, more cautious cultivation of gene-seeds, and tests of purity for both the created organs and the initiates they were to be implanted in. All control of genetic stock was transferred to the Adeptus Terra, on Earth.

In addition, and more noticably, the Space Marine Legions were split up into relatively small Chapters. Smaller Legions like the Space Wolves only split once, whereas the huge Ultramarines Legion was split no less than twenty times. Each would maintain a force of about 1000 warriors.

Finally, rules were laid out in regards to Chapter organization. This included guidelines for the types of squads and characters, as well as the overall structure of the Chapter, in regards to the distribution of these squads across ten companies.

Not all chapters follow the Codex to the letter, nor does the Adeptus enforce such, but most demonstrate an overall faith to the principle.