Old Icelandic relative clauses are frequently preceded by the pronoun sá, considered by most grammars to be a demonstrative. Using a large corpus of Old Icelandic prose, I show that when sá precedes relative clauses, it is often ambiguous between a cataphoric demonstrative (referring ahead to a relative clause) and relative pronoun (part of the relative clause). Syntactic and prosodic evidence indicates that, at least in some instances, sá is unambiguously a relative pronoun, used in tandem with the particle er; thus Old Icelandic relative clauses seem to have doubly filled COMP. A notable characteristic of relative sá is its pervasive attraction to the case of the matrix antecedent. I argue that case attraction represents an intermediate stage in the reanalysis of sá from a demonstrative to a true relative pronoun. Structurally, case-attracting relative pronouns and true relative pronouns occupy different functional positions within a split-CP system. Sá achieved the final stage of the development in the seventeenth century, but rapidly declined under competition with the complementizer sem, thus leaving the false impression that sá never developed beyond the case-attraction stage.