The Primogenitor Chapters are those Space Marine Chapters known to have been split from the Ultramarines after the Horus Heresy in what was known as the Second Founding. These Chapters have stayed close-knit, often fighting alongside each other and always standing ready to answer any call from the Chapter Master of the Ultramarines.[1]
Source
- 1: Codex: Ultramarines (2nd Edition), pg. 9