|Prime Material Plane|
Still think the Prime Material has no interest or influence on the planes? It's this world that gave Sigil the Harmonium (for good or ill)! About 500 years ago, a bunch of cutters here decided to "rid the country of chaos and bring peace to the land." Well, they succeeded, and went on to reshape their entire world in the Harmonium's image. Not satisfied with that, they moved on to the planes, carrying their message of harmony through law with them. Now the Harmonium's one of the most important factions in Sigil, responsible for maintaining the peace and policing the city. The Harmonium maintains a strong relationship with its prime roots; in fact, the current Harmonium factol was born on Ortho. Needless to say, Ortho embodies the Harmonium ideal of peace, harmony, and law - berks who can't stomach these ideals should just avoid this place.
Setting Resource: Factol's Manifesto, pgs. 8, 90-91, 96