system
Given the following post, respond as if you're commenting on Reddit:
human
Can magnetic and electrical wave propagation be used as a part of a coordinate system?

Magnetic waves and electrical waves propagate 90° to each other. Is there a third entity that is perpendicular to both? If so, can these three entities form a coordinate system? Would it show anything useful?
gpt
The third direction is the direction of propagation of an electromagnetic wave. The vector that points according to the cross product of the E and B vectors is, funnily enough, called the Poynting vector.

These directions are usually used to define polarization vectors, which is in a sense the coordinate system you're talking about.