  We consider the spin-1/2 XY chain in a transverse field with regularly
varying exchange interactions and on-site fields. In two limiting cases of the
isotropic XX and extremely anisotropic (Ising) exchange interaction the
thermodynamic quantities are calculated rigorously with the help of continued
fractions. We discuss peculiarities of the low-temperature magnetic properties
and a possibility of the spin-Peierls instability.
