This paper attempts to account for the syntactic distribution of the particle şey in Turkish, in particular its suffixed variant which is a placeholder for expressions that have to be inserted into the discourse later. The paper argues that the distribution of suffixed şey is determined by constituent structure, meaning that şey can only substitute for syntactic constituents. Thus, şey acts as a pro-form, similar, for instance, to pronouns substituting for noun phrases. This has two implications: First, as şey is a quasi-universal pro-form with the ability to substitute for a wide range of constituents, şey-substitution can be used as a constituency test to peek into the constituent structure of virtually any major syntactic domain. Second, the overall şey-substitution pattern across different syntactic domains constitutes evidence for Kayne’s binary branching hypothesis.