  We study pattern selection of cracks in directionally drying fractures by
analyzing the experimental systems recently devised by C. Allain and L. Limat.
[Phys. Rev. Lett. {\bf 74}, 2981 (1995).] Proposing a simple picture of crack
formation, we clarify the mechanism of how cracks array regularly and find that
the interval between neighboring cracks is proportional to the $2/3$ power of
the cell thickness. This result explains well the experimental data of Allain
and Limat.
