When is coupling (induction) between two wires maximum?

Coupling through mutual induction relies on the magnetic field from one wire cutting across another. Magnetic fields around a current-carrying wire form concentric cylinders along its entire length. When two wires are placed close together and parallel, their magnetic fields perfectly overlap and align, allowing for the maximum induction of current from one wire to the other. Conversely, if wires are separated, the magnetic field strength drops off significantly due to distance. If they are placed at right angles to one another, the magnetic field lines cross perpendicularly, which minimizes or entirely eliminates mutual induction. This is why crossing wires at 90 degrees is a common technique used to prevent unwanted interference.