The Semitic interrogative pro-word system includes the element 'ayy-. This element is compatible with different entity types, its functional range across the languages greatly varies, as an interrogative with nominal and/or several adverbial meanings, and it also occurs as a bare form. The pertinent linguistic literature identifies it as an interrogative formative which conveys an interrogative force to adverbs and pronouns. For this idea to be adopted, a principle is devised which accounts for this formal and functional variety of the element 'ayy-, based on comparative Semitic data. The component 'ayy- turns out to be a general constitutent-question marker that is applied to diverse semantic domains (choice/type/quality [~which], time, manner, locative, ablative and allative) to form the corresponding interrogative pro-words.