  We make a thorough comparison between different schemes of merging
fixed-order tree-level matrix element generators with parton-shower models. We
use the most basic benchmark of the O(alpha_S) correction to e+e- -> jets,
where the simple kinematics allows us to study in detail the transition between
the matrix-element and parton-shower regions. We find that the CKKW-based
schemes give a reasonably smooth transition between these regions, although
problems may occur if the parton shower used is not ordered in transverse
momentum. However, the so-called Pseudo-Shower and MLM schemes turn out to have
potentially serious problems due to different scale definitions in different
regions of phase space, and due to sensitivity to the details in the initial
conditions of the parton shower programs used.
