  In this paper, we investigate the lifespan estimates of classical solutions
of the initial value problems for semilinear wave equations of derivative type
with characteristic weights in one space dimension. Such equations provide us
basic principles on extending the general theory for nonlinear wave equations
to the non-autonomous case. In our results, two characteristic weights interact
with each others and produce a different range of parameters on the
global-in-time existence from the nonlinear terms of unknown function itself.
