From Lexicanum

A Founding is an occasion where several new Chapters of Space Marines are created.

Each Founding creates a number of new chapters. The exception to this was the creation of the Grey Knights who were created before the Second Founding.

A founding does not occur at any set interval but is ordered when it is deemed necessary. As chapters are occasionally completely destroyed, Foundings are often meant to ensure that a thousand chapters exist at a time. The latest founding was the Twenty Sixth, in 738.M41.¹

The First Founding occurred at the beginning of the Great Crusade and was the creation of the original Space Marine Legions.

The Second Founding, following the Horus Heresy, was the division of these Legions into the smaller Chapters which remain the Space Marine's foremost unit of organization to the present. One chapter of each parent Legion maintained the original colours, symbol and name of the Legion - these are known as the Chapters of the First Founding.

A "founding" can also refer to the formation or Raising of a new regiment of the Imperial Guard.

Creation of new chapters

The creation of new chapters can only result by order of the High Lords of Terra, and it is said ultimately only from the Emperor himself, although the actual process of creation is performed by the Adeptus Mechanicus.

The gene-seed used to create new chapters is stored by the Adeptus Mechanicus, originally tithed by existing chapters. New progenoids are created using human test-slaves. Zygotes are grown from gene-seed and implanted into the slaves - the zygotes develop into progenoid organs and are removed. A single slave is used in this way to create two progenoids, which are then implanted into two more slaves, and so on. One thousand sets of organs are created, taking over half a century of constant reproduction.

Space Marine Foundings

Founding Number	Date	Details
1st Founding	M29	The creation of the 20 Space Marine Legions.
2nd Founding	021.M31	The huge Legions which remained loyal throughout the Horus Heresy were reorganized into smaller Chapters following the teachings of the Codex Astartes written by Roboute Guilliman.
3rd Founding	001.M32
Unstated Founding	M32	Creation of the Marines Malevolent.
Unstated Founding	M33	Creation of the Angels Porphyr, Howling Griffons, and Sons of Guilliman.
Unstated Founding	598.M35	Creation of the Astral Claws, Executioners and Mantis Warriors.
13th Founding	early M36	The Dark Founding, the only for which the Adeptus Terra does not have a gene-seed sample
19th Founding	M36	Founding of the Avenging Sons
21st Founding	M36	The Cursed Founding created chapters which fell victim to mutation, heresy, rebellion, or simple bad luck.
22nd Founding	M37
Unstated Founding	M37	Creation of the Disciples of Caliban.
Unstated Founding	M38	Creation of the Celestial Lions.
Unstated Founding	M40	Creation of the Omega Marines.
25th Founding	Unstated	Creation of the Star Scorpions.
26th Founding	738.M41	The latest founding to date

Sources

• ¹: White Dwarf 98 Chapter Approved: The Origin of the Legiones Astartes by Rick Priestley (reprinted in Warhammer 40,000: Compendium)
• Codex: Space Marines (3rd Edition)
• Codex: Space Marines (4th Edition)
• Codex: Ultramarines (2nd Edition)
• Index Astartes