  We show that fluctuations of the fireball shape in the longitudinal direction
generate nontrivial rapidity correlations that depend not only on the rapidity
difference, y_{1} - y_{2}, but also on the rapidity sum, y_{1} + y_{2}. This is
explicitly demonstrated in a simple wounded nucleon model, and the general case
is also discussed. We show how to extract different components of the
fluctuating fireball shape from the measured two-particle rapidity correlation
function. The experimental possibility of studying the longitudinal initial
conditions in heavy-ion and proton-proton collisions is emphasized.
