  Single crystals of the Kitaev spin-liquid candidate $\alpha$-RuCl$_3$ have
been studied to determine low-temperature bulk properties, structure and the
magnetic ground state. Refinements of x-ray diffraction data show that the low
temperature crystal structure is described by space group $C2/m$ with a
nearly-perfect honeycomb lattice exhibiting less than 0.2 \% in-plane
distortion. The as-grown single crystals exhibit only one sharp magnetic
transition at $T_{N}$ = 7~K. The magnetic order below this temperature exhibits
a propagation vector of $k$ = (0, 1, 1/3), which coincides with a 3-layer
stacking of the $C2/m$ unit cells. Magnetic transitions at higher temperatures
up to 14~K can be introduced by deformations of the crystal that result in
regions in the crystal with a 2-layer stacking sequence. The best fit symmetry
allowed magnetic structure of the as-grown crystals shows that the spins lie in
the $ac$-plane, with a zigzag configuration in each honeycomb layer. The three
layer repeat out-of-plane structure can be refined as a 120$^o$ spiral order or
a collinear structure with spin direction 35$^o$ away from the $a$-axis. The
collinear spin configuration yields a slightly better fit and also is
physically preferred. The average ordered moment in either structure is less
than 0.45(5) $\mu_B$ per Ru$^{3+}$ ion.
