Syntactic Theory

M-command (Maximal command)

Node c-commands node if and only if

i) does not dominate ; and
ii) every X dominating also dominates , where X is understood as a maximal projection.

M-command is a relaxed version of c-command. Consider the above tree. By the definitions of m-command, we can say that X0 m-commands the following constituents: YP, X' ZP. All we have to do is follow the projection line of X0 up to its maximal projection (XP) and collect all the constituents that XP dominates. This way, we get the m-command domain of X0.
Note that X0 does not m-command WP, since it is the lower segment of XP that is relevant for m-command.