We say that an MMTAG G is in regular form if there are functions ρ i from Σ into the domain of some partial ordering such that for each component α of type X, for each adjunction node η ∈ α, if the jth child of η dominates the ith foot node of α (that is, another component's jth spine would adjoin into the ith spine), then ρ i (X) ρ j (label(η)), and ρ i (X) = ρ j (label(η)) only if η is safe in the ith spine.
it also has much in common with dtree substitution grammar (Rambow et al., 1995).