  We describe a sub-galactic main sequence (SGMS) relating star formation rate
surface density ($\Sigma_{\textrm{SFR}}$) and stellar mass density
($\Sigma_{\star}$) for distinct regions within star forming galaxies, including
their nuclei. We use a sample of 246 nearby star-forming galaxies from the
"Star Formation Reference Survey" and demonstrate that the SGMS holds down to $
\sim $1 kpc scales with a slope of $\alpha=0.91$ and a dispersion of 0.31 dex,
similar to the well-known main sequence (MS) measured for globally integrated
star formation rates (SFRs) and stellar masses. The SGMS slope depends on
galaxy morphology, with late-type galaxies (Sc$-$Irr) having $\alpha = 0.97$
and early-type spirals (Sa$-$Sbc) having $\alpha = 0.81$. The SGMS constructed
from sub-regions of individual galaxies has on average the same characteristics
as the composite SGMS from all galaxies. The SGMS for galaxy nuclei shows a
dispersion similar to that seen for other sub-regions. Sampling a limited range
of SFR$-$M$_{\star} $ space may produce either sub-linearity or super-linearity
of the SGMS slope. For nearly all galaxies, both SFR and stellar mass peak in
the nucleus, indicating that circumnuclear clusters are among the most actively
star-forming regions in the galaxy and the most massive. The nuclear SFR also
correlates with total galaxy mass, forming a distinct sequence from the
standard MS of star-formation. The nuclear main sequence will be useful for
studying bulge growth and for characterizing feedback processes connecting AGN
and star formation.
