Preferred routing direction
Beginner
Each wiring layer runs mostly one way, east–west or north–south, and neighboring layers alternate, like stacked one-way streets.
Novice
The one direction, horizontal or vertical, that wires on a given layer run in. Neighboring layers alternate, so a wire that needs to turn steps up or down a layer through a via. A short sideways piece on a layer (a “wrong-way” step) may be allowed at a high cost or forbidden outright.
Expert
Declared per layer in tech LEF (DIRECTION HORIZONTAL or VERTICAL). On the lowest layers of advanced nodes, the patterning process prints rows of parallel lines, so those layers are effectively one-direction only. Jogs and wrong-way stubs there trigger line-end, minimum-area and mask-coloring rules, so routers keep them for reaching pins.