  By means of electron spin resonance investigations we revealed the crucial
role of the interchain coupling in the spin dynamics of the spin-1/2 Heisenberg
antiferromagnetic (AF) chain material copper-pyrazine-dinitrate,
Cu(C$_4$H$_4$N$_2$)(NO$_3$)$_2$. We found that the dominating interchain
interaction is of a zig-zag type. This interaction gives rise to geometrical
frustration effects and strongly influences the character of AF ordering.
Combining our experimental findings with the results of a quasiclassical
approach we argue that at low temperatures the system orders in an
incommensurate spiral state.
