The languages of the world differ with respect as to whether they allow for infinitival interrogatives and infinitival relative clauses. In order to explain this variation, I postulate the "Wh-Infinitive-Generalization" that links the (non‑)availability of infinitival interrogatives and infinitival relatives to morphological properties of the infinitival C-system. Based on synchronic and diachronic evidence, it is shown that wh-infinitives as well as infinitival relatives are impossible in languages in which the left periphery of the infinitive cannot be occupied by a phonetically realized infinitival complementizer. In contrast, languages with wh-infinitives do exhibit infinitival complementizers. In order to derive the "Wh-Infinitive-Generalization", I argue that infinitival C⁰ is "defective" in languages without wh-infinitives (/ infinitival relatives) where "defective" infinitival C⁰ is understood in analogy to defective T⁰_def (see Chomsky 2000, 2001, 2008), i.e. C⁰_def cannot bear the complete range of features specific for C⁰ (i.e. [focus]-, [wh]-, [topic]-, and [pred]-features). As a consequence, the specifier of C⁰_def like the specifier of T⁰_def may serve only as an intermediate but not as a final landing site of movement.