Why it looks like this

The city's surveyors, laying out land in a gold-rush hurry, ignored the terrain: the grid marches over Nob Hill, Russian Hill and Twin Peaks as if the ground were flat. The result is the famous absurdity of streets at 30-percent-plus grades, cable cars invented (1873) because horses could not haul the hills, and Lombard Street's switchbacks added because the straight block was undrivable.

Market Street cuts diagonally through it all — the seam where two grid systems meet at an angle. The 1906 earthquake and fire destroyed most of the city, but San Francisco rebuilt on exactly the same defiant lines.