Native Lower Sorbian, an endangered West Slavic minority language spoken in Germany, possesses a relative clause formation strategy employing the invariant relativizer ak and optional resumption. The focus of this paper lies on the status of ak. In other languages that have them, invariant relativizers are drawn from the set of complementizers, wh-words, or demonstratives. ak seems to differ in that respect because it belongs to neither category. In this paper, I argue that ak is not an outlier. Instead, ak is a variant of the manner wh-word kak ‘how’ in its non-manner use as a complementizer. After I show how the complementizer kak differs from the wh-adverb kak and that relative clauses in Native Lower Sorbian feature empty operator movement, I argue that the empty operator sitting in SpecCP triggers a rule partially deleting the complementizer kak. More specifically, the rule elides the initial [k] of kak, reducing it to ak. This makes Native Lower Sorbian similar to Bern German or West Frisian, both of which also feature the partial deletion of a complementizer in the presence of a moved element in SpecCP. Furthermore, Native Lower Sorbian is yet another language where how has a non-manner use.
