  I discuss anew how arguments about the internal dynamics of galactic disks
set constraints on the otherwise ambiguous decomposition of the rotation curves
of spiral galaxies into the contributions by the various constituents of the
galaxies. Analyzing the two sample galaxies NGC 3198 and NGC 2985 I conclude
from the multiplicities of the spiral arms and the values of the Q disk
stability parameters that the disks of both galaxies are `maximum disks'.
