Basis is the largest of the 12 moons orbiting the gas giant planet Threshold in the Soell system. Its diameter is 23,848 km (14,818 miles), almost double that of Earth, but with the same surface gravity. Installation 04 was positioned in the L1 Lagrange point between Basis and Threshold. After Installation 04's destruction, the Covenant Heretics built a makeshift camp on Basis amid its wreckage.
The atmosphere of the moon produces a twilight appearance, with hazy clouds and an orange skyline.
- On Burial Mounds, you can see wreckage from Installation 04 falling into the atmosphere and burning up.
- The Arbiter was originally meant to take place on Basis instead of the Gas Mine on Threshold.
- In the first cinematic of Halo: Combat Evolved, as the Pillar of Autumn is hit by several Plasma Torpedoes and the camera pivots around the ship, you can catch a glimpse of the Basis.
- In Halo: Combat Evolved, Basis appears as a image of Mars, tinted blue.
- The blue coloring of Basis when seen from space is puzzling, considering that from the surface it has a hazy orange or rust-colored atmosphere and terrain. However, it is possible that this all from the scoring and burning of Halo's wreckage.
- Halo: The Fall of Reach (First appearance)
- Halo: Combat Evolved
- Halo 2