Why does transistor orientation matter in analog layout?

Transistor orientation matters a lot in analog layout because it affects matching. For example, when we design a current mirror, if one transistor is rotated differently from the other, their stress, well proximity effect, or even variation in diffusion can change. This will cause mismatch, and the mirror won’t give accurate current. So, to keep symmetry and matching, I always try to keep the same orientation for transistors.