It is the plane of ultimate law and order (neutral or chaotic creatures take -2 penalty on Charisma, Intelligence and Wisdom-based checks), where the souls of people of lawful neutral alignment go after death. Mechanus operates on a strict schedule where every action is planned, measured and controlled perfectly. It is home to the construct-like geometric modrons, the law-enforcing inevitables and the regimented giant ants called formians.
Throughout Mechanus exists a series of interlocking cogs that float in space, intersecting at all angles and directions. Many of the cogs are gigantic, stretching many miles across and turn so slowly that the rotation is undetectable. Smaller cogs can move at much higher speeds, but no acceleration is experienced, except at the very edges. Usually smaller cogs bunch up with larger ones; these are called knots. The gears are fiercely guarded by gear spirits that reside within them. The gravitational force is oriented towards the face of each individual cog, making intercog travel a disconcerting experience for the uninitiated.
Regulus
Regulus is the home of the construct-like Modrons. Their strict hierarchy originates from Primus, a greater deity, which resides here. Regulus consists of a total of sixty-four cogs, a number that never varies. It is divided into four quarters of sixteen cogs, each ruled by a Secundus. Each quarter is in turn split into four regions, of four cogs, each ruled by a Quarton. Finally, the regions are further subdivided into four sectors, of a single cog each, ruled by an Octon.
The Center
At the center of the Formian realm lies a central cog more than three-thousand miles across, where the Scion Queen Mother, leader of the Formians resides. The central cog supposedly imparts the movement to all other cogs in Mechanus.
Neumannus
Possibly named after John von Neumann, it is a series of mysterious self-sustaining factories that produce Inevitables. It is managed and presided by the Hub of Elders, who seeks out any non-lawful activity (broken contracts, etc.) that needs to be corrected.