  The spin-1/2 J1-J2 antiferromagnet is a prototypical model for frustrated
magnetism and one possible candidate for a realization of a spin liquid phase.
The generalization of this system on the anisotropic square lattice is given by
the J1-J2-J1'-J2' Heisenberg model, which can be treated by a renormalization
group (RG) analysis of coupled frustrated chains. The J1-J2-J1'-J2'-model shows
an interesting interplay of N\'eel order, dimerization, and spin liquid
behavior. The analytical findings supported by numerical results indicate that
for the isotropic model the phase at intermediate coupling strength 0.4 J1 < J2
< 0.6 J1 is not a spin liquid, but has instead a spontaneously staggered dimer
order.
