Still more types of particles occurr only in main clauses. These include a set of "initial particles", some modal in meaning [...], most functioning as sentence connectives [...]. It is these particles, mentioned in Zwicky 1977b [Zwicky, Arnold M., 1977, On clitics, Bloomington, Indiana University Linguistic Club] that appear to constitute an exception t Kaisse's version of Wackernagel's Law-IF they are clitics. The initial particles would also constitute straightforward exceptions - again, IF they are clitics. But there is no reason to think the final and initial particles are anything other than words: adverbs, in fact.
- Zwicky (2004), Pag. 367