  TThe parent compounds of iron-arsenide superconductors, $A$Fe$_{2}$As$_{2}$
($A$=Ca, Sr, Ba), undergo a tetragonal to orthorhombic structural transition at
a temperature $T_{\mathrm{TO}}$ in the range 135 to 205K depending on the
alkaline earth element. Below $T_{\mathrm{TO}}$ the free standing crystals
split into equally populated structural domains, which mask intrinsic,
in-plane, anisotropic properties of the materials. Here we demonstrate a way of
mechanically detwinning CaFe$_{2}$As$_{2}$ and BaFe$_{2}$As$_{2}$. The
detwinning is nearly complete, as demonstrated by polarized light imaging and
synchrotron $X$-ray measurements, and reversible, with twin pattern restored
after strain release. Electrical resistivity measurements in the twinned and
detwinned states show that resistivity, $\rho$, decreases along the
orthorhombic $a_{o}$-axis but increases along the orthorhombic $b_{o}$-axis in
both compounds. Immediately below $T_{\mathrm{TO}}$ the ratio $\rho_{bo}/
\rho_{ao}$ = 1.2 and 1.5 for Ca and Ba compounds, respectively. Contrary to
CaFe$_{2}$As$_{2}$, BaFe$_{2}$As$_{2}$ reveals an anisotropy in the nominally
tetragonal phase, suggesting that either fluctuations play a larger role above
$T_{\mathrm{TO}}$ in BaFe$_{2}$As$_{2}$ than in CaFe$_{2}$As$_{2}$, or that
there is a higher temperature crossover or phase transition.
