We investigate low-energy properties of a generalized spin ladder mode
l with both of the spin alternation and the bond alternation, which al
lows us to systematically study not only ladder systems but also alter
nating spin chains. By exploiting non-linear a model techniques we stu
dy the model with particular emphasis on the competition between gapfu
l and gapless states. Our approach turns out to provide a more consist
ent semi-classical description of alternating spin chains than that in
the previous work. We also study a closely related model, i.e., a spi
n chain with plaquette structure, and show that frustration causes lit
tle effect on its low-energy properties so far as the strength of frus
tration is weaker than a certain critical value.