The strong corrugation of the V4O5 layer in the spin gap system CaV4O9 is examined in this paper for its impact on the exchange coupling constants between the spin 1/2 V ions. Local spin density calculations show that the V spin occupies a dx2-y2 orbital (x and y are the V-V directions) that would have vanishing 2nd order exchange coupling if the V4O5 layer were flat. The Kanamori-Goodenough superexchange rules, and the small calculated energy difference of ferromagnetic and antiferromagnetic alignments of spins for the real structure, indicate that 2nd neighbor couplings dominate over nearest neighbor. This suggests two coupled S=1/2 metaplaquette systems, each tending toward singlet formation. This new picture has been confirmed by neutron scattering measurements of the singlet-triplet excitation spectrum and by subsequent theoretical work.