For a TSL 2 grammar G = T, S , T ⊆ Σ will be referred to as the tier, S the allowed tier substrings, and R = fac T-2 (Σ * ) − S as the forbidden tier substrings.
let H = Σ − T be the non-tier elements of G. For any elements σ i , σ j ∈ T , their tier adjacency with respect to a set l of strings refers to whether or not they may appear adjacent on the tier; formally, σ i , σ j are tier adjacent in l iff An exclusive blocker is a σ ∈ T which does not appear in R. ∀w ∈ R, σ ∈ fac 1 (w).