From Lexicanum

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

A founding does not occur at any set interval but is ordered when it is deemed necessary by the High Lords - the creation of new chapters is completely in the hands of the High Lords as a Founding can only be ordered by the Senate of High Lords of Terra. 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.

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.

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.

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

Space Marine Foundings

Sources

• Warhammer 40,000: Compendium, The Origin of the Legiones Astartes
• Codex: Space Marines (3rd Edition)
• Codex: Space Marines (4th Edition)
• Codex: Ultramarines (2nd Edition)
• Index Astartes