Admissibility in propositional logics is closely related to unification in the equational theory of modal or Heyting algebras. The connection was developed by Ghilardi (1999, 2000). In the logical setup, a unifier of a formula "A" in a logic "L" (an "L"-unifier for short) is a substitution σ such that σ"A" is a theorem of "L". (Using this notion, we can rephrase admissibility of a rule "A"/"B" in "L" as "every "L"-unifier of "A" is an "L"-unifier of "B"".) An "L"-unifier σ is less general than an "L"-unifier τ, written as σ ≤ τ, if there exists a substitution υ such that