  Theoretical arguments and large-scale two-fluid simulations are used to study
the spreading of reconnection X-lines localized in the direction of the current
as a func- tion of the strength of the out-of-plane (guide) magnetic field. It
is found that the mech- anism causing the spreading is different for weak and
strong guide fields. In the weak guide field limit, spreading is due to the
motion of the current carriers, as has been pre- viously established. However,
spreading for strong guide fields is bi-directional and is due to the
excitation of Alfv\'en waves along the guide field. In general, we suggest that
the X-line spreads bi-directionally with a speed governed by the faster of the
two mecha- nisms for each direction. A prediction on the strength of the guide
field at which the spread- ing mechanism changes is formulated and verified
with three-dimensional simulations. Solar, magnetospheric, and laboratory
applications are discussed.
