Orientation of the reconnection X‐line

We propose a criterion for identifying the orientation of the X‐line when two regions of plasma with arbitrary densities, temperatures, and magnetic fields undergo reconnection. The X‐line points in the direction that maximizes the (suitably‐defined) Alfvén speed characterizing the reconnection outflow. For many situations a good approximation is that the X‐line bisects the angle formed by the magnetic fields.