Comment clauses and the emergence of new discourse markers: Spanish lo que es más

• Comment clauses are a step in the process of discourse markers grammaticalization.
• Evaluation and argumentation are interconnected in the creation of comment clauses.
• Lo que es más is not a discourse marker in Spanish (the elaborative marker is es más).
• Lo que es más + adjective is a free evaluative construction, a comment clause.
• Lo que es mejor/lo que es peor are argumentative operators.

This study focuses on a construction, lo que es más, whose English equivalent, what's more, has been regarded as a “comment clause” ( Brinton, 2008) or a “conjunct” (Quirk et al., 1972), implying that it belongs to the category of “discourse marker”. Having traced the development of lo que es más in the history of Spanish, the author argues that the construction is still in the process of evolving: a free syntactic construction (lo que es más + adjective), a parenthetical with evaluative meaning, coexists with two argumentative operators (lo que es mejor and lo que es peor) and the elaborative connectives lo que es más and es más, this latter is found to be more frequent in speech, according to the corpus. The evolution of lo que es más helps us to understand the grammaticalization of discourse markers, and the syntactic function of comment clauses and parentheticals expressing speaker evaluation, as well as the hierarchy of the arguments in the text.