  We report measurements of the Gaussian contribution, T_{2G}, to the plane
^{63}Cu nuclear spin--spin relaxation time in the YBa_2Cu_3O_7 and YBa_2Cu_4O_8
blocks of normal and superconducting Y_2Ba_4Cu_7O_{15}. The data confirm our
previous results that adjacent CuO_2 planes have different doping levels and
that these planes are strongly coupled. -- The static spin susceptibility at
the anti-ferromagnetic wave vector exhibits a Curie--Weiss like temperature
dependence in the normal state. -- The Y_2Ba_4Cu_7O_{15} data are incompatible
with a phase diagram based on a single CuO_2 plane theory and suggest that the
appearance of a spin gap implies interplane coupling. Additional data for
YBa_2Cu_4O_8 and YBa_2Cu_3O_{6.982} are in accord with the single plane theory.
-- The temperature dependence of T_{2G,ind} below T_c excludes isotropic s-wave
superconductivity in all three compounds.
