We define a notion of relevance of a clause for proving a par-
ticular entailment by the resolution calculus. We think that our notion of
relevance is useful for explaining why an entailment holds. A clause is rel-
evant if there is no proof of the entailment without it. It is semi-relevant
if there is a proof of the entailment using it. It is irrelevant if it is not
needed in any proof. By using well-known translations of description log-
ics to first-order clause logic, we show that all three notions of relevance
are decidable for a number of description logics, including EL and ALC.
We provide effective tests for (semi-)relevance. The (semi-)relevance of a
DL axiom is defined with respect to the (semi-)relevance of the respective
clauses resulting from the translation.