  We present a unifying theory for the observed complex structures of the
sp-bonded elements under pressure based on nearly free electron picture (NFE).
In the intermediate pressure regime the dominant contribution to crystal
structure arises from Fermi-surface Brillouin zone (FSBZ) interactions -
structures which allow this are favoured. This simple theory explains the
observed crystal structures, transport properties, the evolution of internal
and unit cell parameters with pressure. We illustrate it with experimental data
for these elements and ab initio calculation for Li.
