A Chapter is a completely autonomous military unit made up of one thousand of the superhuman warriors known as Space Marines or Astartes and their related vehicles, starships and support personnel. Each Chapter serves collectively with the others of its kind as part of the Imperium of Man's Adeptus Astartes. Every Chapter is entirely autonomous from every other Adepta of the Imperium, including the Inquisition, and takes its orders only from the High Lords of Terra themselves. Most Chapters have feudal title to an entire planet of the Imperium which serves as its Chapter homeworld or to a large fleet of powerful and massive starships that serves as a mobile headquarters if the Chapter is fleet-based. The majority of Space Marine Chapters follow the dictates of the Codex Astartes, a sacred tome written by the Ultramarines Primarch Roboute Guilliman. The Codex Astartes details the proper organisational structure and mobilisation methods for a Space Marine Chapter. The Codex has been adopted by the vast majority of Space Marine Chapters as their organisational blueprint. Consequently, most Space Marine Chapters are considered "Codex Chapters" or "Codex-compliant Chapters" due to their adherence to the Codex Astartes.
During the Great Crusade in the late 30th Millennium, the Space Marines were originally organised into 20 Legions each of which was further divided into companies. The XIII Legion, the Ultramarines, led by Roboute Guilliman, became the largest of all the Astartes Legions as a result of Guilliman's tactical mastery and a steady flow of new recruits from the Realm of Ultramar. The Ultramarines became so large, growing to a size that included over 100,000 Astartes, that a new, larger unit of division, the Chapter, was created to better organise the XIII Legion's forces. These Chapters (alternatively designated as Great Companies, Harrows, Millennials, etc.) were originally composed of roughly 1,000 line Legionaries, but as the Legions grew, this number began to vary substantially by Legion, and also through the vicissitudes of war and the availability of replacement recruits. At the time of the outbreak of the Horus Heresy in the early 31st Millennium, the Ultramarines Legion was divided into twenty Chapters, each composed of ten companies, with each company composed of roughly one thousand Astartes.
Following the end of the Heresy, Guilliman, as the new Lord Commander of the Imperium, ordered during the event known as the Second Founding that the remaining Loyalist Space Marine Legions be divided and re-organized into smaller one-thousand Astartes Chapters, to ensure that no future rebel such as Horus could gain control of such a large and power military unit as a Space Marine Legion again. Newly formed Chapters created at this time that were carved out of the remaining 9 Loyalist Legions became known as Successor Chapters.
As a result of this change in organisation, the Space Marines' military purpose changed as well. Where the Space Marine Legions had once served as the Imperium's primary frontline military forces, that role was given over to the newly created Imperial Guard, which had been created from the remains of the Imperial Army in the wake of the Heresy. The Imperial Army had been used primarily as a reserve force of garrison troops during the Great Crusade, but its successor would now bear the primary burden of the Imperium's defence. The Space Marine Chapters were now intended to primarily serve as highly mobile planetary assault and special forces troops, who would be deployed only for the toughest missions confronting the Imperium for which the forces of the Imperial Guard were simply insufficient.
New Space Marine Chapters are not created piecemeal as required by the Imperium's strategic needs, but rather in deliberate groupings called "Foundings." The process by which a new Founding's creation is approved by the Imperial government is mysterious and arcane, subject to decades or even centuries of planning before it is announced. It is only by an edict of the High Lords of Terra that such an undertaking as the creation of new Chapters can be instigated, for it requires the cooperation and mobilisation of countless divisions within the Imperium's monolithic and vast governmental organisations. Establishing new Astartes Chapters on an individual basis is nigh impossible--the mobilisation of such vast resources is beyond the ability of any single segment of the Imperium.
The Adeptus Mechanicus plays an essential role in the process of a Founding, for its highest echelons are tasked with creating, testing and developing the gene-seed samples that will provide the genetic foundation of the new Chapters. By ancient custom, the Mechanicus has the right to expect a tithe of 5% of the gene-seed of every Space Marine Chapter to assist it in the creation of new Chapters and to check that the purity of the existing Astartes Chapters' DNA has remained untainted by mutation or exposure to Chaos. Each Chapter is created from the gene-seed of an existing donor Chapter. The zygote derived from each type of gene-seed is implanted by the Magi Biologis of the Adeptus Mechanicus in a human test-slave who spends his entire life in a static experimental capsule, immobile and serving as nothing but a medium from which two Progenoid Glands will develop. When the Progenoids are fully developed, they are extracted from the original test-slave and then implanted into another two test-slaves, producing four Progenoids, and so on. It takes 55 standard years of this type of reproduction to create a healthy set of 1,000 new Astartes organs. These must be tested for purity and genetic consistency before they will be sanctioned officially by the Fabricator-General of the Adeptus Mechanicus and then by the High Lords of Terra, speaking for the Emperor of Mankind, who alone can give permission for the creation of a new Chapter.
Entire Forge Worlds may be turned over to the manufacture of the mighty arsenal of weaponry, ammunition, Power Armour, vehicles and starships that any such force will require. There are a myriad of other concerns as well. A suitable homeworld inhabited by humans must be identified for the new Chapter, which will likely provide not only a secure and defensible base of operations, but also a source of new recruits as well. Such worlds might have been reported by itinerant Rogue Traders and earmarked centuries before by Adeptus Mechanicus Explorators as potential Astartes homeworlds. A degree of environmental terraforming might be required and the natives of the world (if they are to become the source of the new Chapter's Aspirants) must be studied and tested by the Mechanicus' Magi Biologis and Genetors for many generations to ensure they are genetically pure and free of any strain of mutation that might later affect the Chapter itself. The construction of a Chapter's fortress-monastery may be one of the greatest undertakings of all, drawing on the genius of the Imperium's most accomplished military architects and engineers. If the Chapter is to be fleet-based, then even more work must be put into the construction of a massive Chapter Barque or an unusually large Battle Barge to serve as the Chapter's mobile fortress-monastery and all of the related capital warships and Escorts such a highly-mobile Chapter will require.
The already extant Space Marine Chapters may also have a role in this process, though to what degree can vary greatly from Founding to Founding. Many of the First Founding Chapters maintain close links with Chapters created using their own gene-seed stocks, and the Chapter Masters might have a hand in planning future Foundings using that genetic material. It is said that the Disciples of Caliban, a Dark Angels Successor Chapter, was created following the direct appeal of the Supreme Grand Master of the Dark Angels, an extremely rare request.
In the more than 10,000 standard years that have passed since the First Founding of the 20 original Space Marine Legions by the Emperor, there have been 25 subsequent Foundings of new Chapters of the Adeptus Astartes; with the most recent, the 26th Founding, occurring in the year 738.M41, approximately 250 standard years ago. Even before a new Founding is announced, entire generations of Imperial servants may have toiled in preparation. Even once the process has been declared and is underway, it is likely to be at least a standard century before the new Chapters are ready to begin combat operations. In times of dire need for the Imperium, faster development has been attempted, but this has often resulted in disaster. Gene-seed cultured in haste is likely to degrade or to mutate, and a great many other factors can lead the entire process astray. And there is no foe more dangerous to the Imperium of Man that a Space Marine who has been corrupted by Chaos or gone Renegade for another reason.
Each Codex-compliant Space Marine Chapter consists of 1000 fighting Space Marines plus support staff which are divided into 10 companies. Each Chapter is led by a Chapter Master. Unlike the Imperial Guard, the Adeptus Astartes has no leadership above the Masters of the individual Chapters. Also unlike many other Imperial military forces, the Astartes has no members to represent the organization as a High Lord of Terra on the Senatorum Imperialis. Besides the Space Marine fighting brethren, there are many non-combat personnel, fully part of the Chapter but generally not involved directly in combat. These include the Chapter's hereditary slaves or bonded Chapter Serfs, Astropaths, Navigators, etc. Each company of a Codex-compliant Chapter has its own command staff, consisting of a:
Most companies will usually be accompanied by a Command Squad of five Veteran Space Marines that will include:
- Company Captain
- Company Champion
- Apothecary - For a Space Marine Chapter to have a future it must carefully protect and preserve the organic implants created from gene-seed that turn an initiate into a full-fledged Space Marine. An Apothecary serves as a Chapter medic and the overseer of the Chapter's gene-seed and implantation process for the creation of new Space Marines.
- Standard Bearer - Standard Bearers carry the company banner that shows the company's feats and prowess. Captains may also display their personal heraldry in the banner. On rare occasions, the Chapter Banner might be fielded by a particular company's Command Squad.
The 1st Company of any Codex-compliant Space Marine Chapter is made up of Veteran Marines. These skilled warriors are the only members of the Chapter trained in the use of Terminator Armour. The 2nd to 5th Companies of the Chapter are referred to as Battle Companies. Each Battle Company consists of:
- Six 10-man Tactical Squads
- Two 10-man Assault Squads
- Two 10-man Devastator Squads
- Support tanks, Assault Bikes, Land Speeders and Rhino armoured transports
- Dreadnoughts - Dreadnoughts are revered warriors who are mortally wounded and entombed in a cybernetic Dreadnought sarcophagus which serves as a heavy support combat walker
The 6th to 9th Companies of a Space Marine Chapter are its Reserve Companies. The 10th Company of a Space Marine Chapter is the Scout Company composed of new Space Marines who are being exposed to combat for the first time. Those Scout Marines who survive this tour of duty are welcomed to the Chapter as full Initiates of the Chapter and are granted a suit of Power Armour. They are them moved into one of the Reserve Companies to begin climbing the Chapter's hierarchy -- if they survive.
Codex Astartes-Deviant Chapters
Notable Chapters that do not follow the organizational standards of the Codex Astartes include:
These Chapters' unique structures, orders of battle and histories are detailed on their relevant pages.
- Codex Imperialis, pp. 16-17
- Codex: Space Marines (6th Edition), pp. 8-9, 14-15
- Codex: Space Marines (5th Edition), pp. 13-14
- Index Astartes I, "Codex Astartes: The Holy Tome of the Space Marines", by Rick Priestley & Andy Chambers, pp. 13-16
- Warhammer 40,000 Rulebook (6th Edition), pg. 183
- Know No Fear (Novel) by Dan Abnett