The mono-valence VO₂ and V₂O₃ and the mixed-valence V_nO_2n−1(n=3−6,8,9)${\mathrm{V}}_{\mathrm{n}}{\mathrm{O}}_{2\mathrm{n}-1}(\mathrm{n}=3-6,8,9)$ exhibit first-order metal-insulator transitions (MIT).

The relative roles of the Mott-like or Peierls-like interactions in these MIT were under debate for several decades.

An unexpectedly simple systematics of T_MIT$T_{\mathrm{MIT}}$ is presented here: For T_MIT=|T_M−T_P|$T_{\mathrm{MIT}}=|T_M-T_P|$ all data points of T_M−T_P$T_{\mathrm{M}}-T_{\mathrm{P}}$ versus 1/n lie on, or close to, two simple straight lines.