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Though not extensive, it encompasses a region of space some three hundred light years in diameter, with the Tau homeworld at its centre, and just over a hundred settled worlds. A number of these worlds are home to alien races which are either subservient to the Tau or whose service are bought.
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	The Tau Empire encompasses a dense yet astrographically small area of space. Many hundreds of star systems exist within this region, and an unusually high proportion harbour an environment conducive to life. As the Tau steadily expand the borders of their empire, they continue to encounter other races. The empire now encompasses over twenty septs - fully developed Tau systems - and a large number of vassal alien homeworlds. The populations of these worlds are fully integrated in to the empire

Tau Codex