- "And henceforth be it known that the Imperium shall be divided into five fleet zones to be known as the Segmentae Majoris. These shall each have a Segment Fortress, from whence the orders of his glorious magnificence, the Emperor, shall be enacted. Imperialis Command Decoriatum shall reside in each and with just duty be issued over the realms within their boundaries. For the Segmentum Solar, this shall be Mars. World of the Adeptus Mechanicus and this Segment shall extend about Mars. In the north of our most majestic domains shall be Segmentum Obscurus, being in fiefdom to the world of Cypra Mundi. To the South it shall make Segmentum Tempestus, with overlordship residing with the planetary estates of the hereditary Bakka. To the West shall be Segmentum Pacificus, under the auspices of the Lords of Hydraphur. The most turbulent East, a region of much strife and dissension and of many lost worlds shall have its overlordship known as Ultima Segmentum, and its guarantor of safety shall be on the planetary Governor Kar Duniash. The Segment Fortress of the Imperial official of the Administratum shall be known as the Master of the Segmentum Solar. Lastly, the region of Segmentum Pacificus, and the Master of the Ultima Segmentum. In this the Emperor is most benevolent and thus shall the Emperor's charges be authority over such domains and reside with those who have the faith and temperament to dictate to the will of those others and take from them that which is the Emperor's due."
- — Imperial Decree in M31 creating the Segmentae Majoris
A Segmentum (pl. Segmentae) is the largest sub-unit of Imperial space and there are five segmentums covering Imperial territory stretching across the entire Milky Way Galaxy. They are the central Segmentum Solar, the eastern Segmentum Ultima, the southern Segmentum Tempestus, the northern Segmentum Obscurus and the western Segmentum Pacificus. Each Segmentum is divided into multiple Sectors. The size of a Sector varies according to local demands and stellar density. A typical sector might encompass 7 million cubic light years, equivalent to a cube with sides almost 200 light years long. Sectors are divided into Sub-sectors, usually comprising between 2 to 8 star systems within a 10-light-year radius (some may encompass more systems -- others only a single solar system). This size is governed by the practical patrol ranges through the Warp of Imperial Navy starships. Because Sub-sectors are divisions of worlds (rather than volumes of space) there are vast numbers of star systems within each sector which do not fall within a Sub-sector. These are referred to as Inter-sectors - and are more commonly called wilderness zones, forbidden zones, empty space and frontier space. Inter-sectors may contain gas or dust nebulae, inaccessible areas, alien systems, unexplored systems, uninhabited systems and uninhabitable worlds.
|Segmentum||Area||Segmentum Fleet Headquarters|
|Segmentum Solar||Galactic Center||Mars|
|Segmentum Ultima||Galactic East||Kar Duniash|
|Segmentum Tempestus||Galactic South||Bakka|
|Segmentum Obscurus||Galactic North||Cypra Mundi|
|Segmentum Pacificus||Galactic West||Hydraphur|
- Battlefleet Gothic Rulebook, pg. 86
- Dark Heresy Core Rulebook (RPG), pp. 252-253
- Warhammer 40,000 Rulebook (5th Edition)
- White Dwarf 139 (UK), "Interstellar Travel"
- White Dwarf 140 (UK), "Interstellar Travel"