  The present paper studies the influence of the structural modulation on the
low energy physics of the $Sr_{14-x}Ca_xCu_{24}O_{41}$ oxides, using ab-initio
determination of the on-site and nearest neighbor effective parameters. The
structural modulations appears to be the key degree of freedom, responsible for
the low energy properties, such as the electron localization, the formation of
dimers in the $x=0$ compound or the anti-ferromagnetic order in the $x=13.6$
compound.
