  The parent compounds of recently discovered iron-arsenide superconductors,
$A$Fe$_2$As$_2$ with alkaline earth $A$=Ca, Sr, Ba, undergo simultaneous
structural and magnetic phase transitions at a temperature $T_{SM}$. Using a
combination of polarized light microscopy and spatially resolved high-energy
synchrotron X-ray diffraction we show that the orthorhombic distortion leads to
the formation of 45$^o$-type structural domains in all parent compounds.
Domains penetrate through the sample thickness in the $c$- direction and are
not affected by crystal defects such as growth terraces. The domains form
regular stripe patterns in the plane with a characteristic dimension of 10 to
50 $\mu m$. The direction of the stripes is fixed with respect to the
tetragonal (100) and (010) directions but can change by 90$^o$ on thermal
cycling through the transition. This domain pattern may have profound
implications for intrinsic disorder and anisotropy of iron arsenides.
