  A simple but useful guide for understanding the structure of light nuclei is
presented. It is based on counting the number of interacting pairs in different
spin-isospin (S,T) states for a given spatial symmetry, and estimating the
overall binding according to the sum of sigma_i.sigma_j tau_i.tau_j expectation
values, as suggested by one-pion-exchange. Applied to s- and p-shell nuclei,
this simple picture accounts for the relative stability of nuclei as A
increases and as T changes across isobars, the saturation of nuclear binding in
the p-shell, and the tendency to form d, t, or alpha subclusters there. With
allowance for pairwise tensor and spin-orbit forces, which are also generated
or boosted by pion-exchange, the model explains why mixing of different spatial
symmetries in ground states increases as T increases across isobars, and why
for states of the same spatial symmetry, the ones with greater S are lower in
the spectrum. The ordering of some sd-shell intruder levels can also be
understood. The success of this simple model supports the idea that
one-pion-exchange is the dominant force controlling the structure of light
nuclei.
