Large-scale magnetic fields in spiral galaxies are strongest in spiral
-shaped regions, the magnetic arms. It was recently discovered for the
galaxy NGC 6946 that magnetic arms can be interlaced with the gaseous
arms, rather than coinciding with them. There are indications that th
e magnetic and gaseous arms may cross in some other galaxies. We sugge
st that magnetic arms can be located away from regions of higher gas d
ensity (gaseous arms) because interstellar turbulence is stronger in t
he latter. We predict magnetic arms interlaced with gaseous arms in ga
laxies with weak dynamos, whereas the two spiral structures should ove
rlap in galaxies with strong dynamo action; in an intermediate case, t
he magnetic and gaseous spiral structures overlap in the inner galaxy
and are interlaced in the outer parts (as, possibly, in M51). Another
plausible mechanism to produce displaced magnetic and gaseous spiral p
atterns results from a delay in the dynamo response to the enhancement
of turbulence in the gaseous arms. This should lead to the magnetic a
nd gaseous arms crossing at the corotation radius, as possibly observe
d in the galaxies IC 342 and M83. We also argue that spiral arms only
weakly affect the local scaleheight of the galactic gas layer.