  In this paper we discuss $3d$ ${\cal N}=2$ supersymmetric gauge theories and
their IR dualities when they are compactified on a circle of radius $r$, and
when we take the $2d$ limit in which $r\to 0$. The $2d$ limit depends on how
the mass parameters are scaled as $r\to 0$, and often vacua become infinitely
distant in the $2d$ limit, leading to a direct sum of different $2d$ theories.
For generic mass parameters, when we take the same limit on both sides of a
duality, we obtain $2d$ dualities (between gauge theories and/or
Landau-Ginzburg theories) that pass all the usual tests. However, when there
are non-compact branches the discussion is subtle because the metric on the
moduli space, which is not controlled by supersymmetry, plays an important role
in the low-energy dynamics after compactification. Generally speaking, for IR
dualities of gauge theories, we conjecture that dualities involving non-compact
Higgs branches survive. On the other hand when there is a non-compact Coulomb
branch on at least one side of the duality, the duality fails already when the
$3d$ theories are compactified on a circle. Using the valid reductions we
reproduce many known $2d$ IR dualities, giving further evidence for their
validity, and we also find new $2d$ dualities.
