Simple cubic structure with one atom in the unit cell found in compressed calcium is contrintuitive with regards to traditional view on tendency of transition to densely packed structures on the increase of pressure. To understand this unusual transformation it is necessary to assume electron transfer from outer core to the valence band and increase of valence electron number for calcium from 2 to ~3,5. This assumption is supported by the model of the Fermi sphere - Brillouin zone interaction that increases under compression. Recently found structure of Ca-VII with tetragonal cell containing 32 atoms (tI32) is similar to the intermetallic compound In5Bi3 with 3,75 valence electrons per atom. Structural relations are analyzed in regard to a resemblance of the electronic structure. Correlations of structure and physical properties of Ca are discussed.