Tocharian A and B are two closely related extinct languages, formerly spoken in the Tarim Basin (present-day China). They are sometimes referred to as East Tocharian (A) and West Tocharian (B), respectively. In both Tocharian A and B the interrogative pronoun ‘what, who’ is used to mark relative clauses. Although the paradigm is somewhat reduced in comparison to that of other pronouns and nouns, it does inflect for case, but not for number or gender. The pronoun is composed of the root ku- and inflected forms of the proximal demonstrative su, see Table 5. The former part comes from PIE *kwi-/kwo-, while the latter goes back to the PIE demonstrative *so-/to-. In Tocharian A, an element ne is added in relative clauses, but it does not necessarily appear directly after kus (indicated by a = in the table). – In addition, there are attributive interrogative-relative pronouns (‘which’) in both languages. There are gaps in the paradigm due to limited attestation, and one of them is only found in Tochar- ian B (see Table 6). Their origin is unclear, but they also consist of the demonstrative su as their second part (Krause & Thomas 1960: 165-167). – Although ku-se and ku-s=ne do appear in other subordinate clauses apart from relative ones, this is a relatively late development in both languages and remains rare. Moreover, they almost never appear in complement clauses (Hackstein 2012: 120).