The cover picture shows superimposed sections from ball-and-stick and space-filling models of the first stable 1,3-diphosphacyclobutane-2,4-diyl (Ar-PC-Cl)2 (Ar = 2,4,6-tBuC6H2; P violet, Cl green, C black, H white). The structure of the molecule is characterized by a planar four-membered ring and trans substituents, and it can be considered as an opened form of the corresponding bicyclobutane. According to ab initio calculations the singlet ground state of the molecule is stabilized by interactions of the unpaired electrons on the C atoms with the lone pairs on the P atoms. More about this theoretically interesting heterocycle is reported by E. Niecke et al. on pp. 555ff. The cover picture was produced by U. Haberl on an IBM workstation with the Schakal program.