The so-called width law for Liesegang patterns, which states that the positions x(n) and widths w(n) of bands verify the relation x(n)similar to w(n)(alpha) for some alpha>0, is investigated both experimentally and theoretically. We provide experimental data exhibiting good evidence for values of alpha close to 1. The value alpha = 1 is supported by theoretical arguments based on a generic model of reaction diffusion.