 CL is an archetypical term rewriting system (TRS). These systems comprise a wide range of formal calculi from syntactic specifications of programming languages and context-free grammars to Markov algorithms; even some number theoretic problems may be viewed as special instances of questions about TRSs. Several notions and proof techniques that were originally invented for CL, later turned out to be useful in applications to less well-understood TRSs.