A criterion of reducibility for a parallelohedron

A parallelohedron is called reducible, if it can be represented as a direct product of two parallelohedra of lower dimension. In his Ph.D. thesis (2005) the first author proved a criterion of reducibility of a parallelohedron in terms of the Venkov graph. In December 2011 the second author presented a slightly revised version of the original proof at the seminar "Discrete Geometry and Geometry of Numbers" (Moscow State University). The present paper follows that talk.