Two distinguishable mechanisms of the monoclinic-to-tetragonal transition in VO 2 are identified: an antiferroelectric-to-paraelectric transition at a temperature Tt as well as a change from homopolar to metallic VV bonding at a temperature Tt'. In pure VO 2 at atmospheric pressure, the two transitions occur at the same temperature, Tt' = Tt = 340°K. However, a Tt' < Tt may be induced by atomic substitutions; and in the intermediate temperature range the structure is orthorhombic, the antiferroelectric distortions being somewhat different in the absence of homopolar VV bonding. From energy-band considerations, the semiconductor-to-metal transition is to be associated with the transition at Tt; but below Tt' the semiconducting energy gap should be larger and the charge-carrier mobilities should be smaller. The existence of two transition temperatures in doped VO 2 is distinguished from the claim of two transition temperatures in the Magnéli phases V nO 2 n-1 . In this latter case, the appearance of two transitions reflects a two-phase region consisting of two adjoining Magnéli phases.