This article presents a case study of Czech that opens the possibility of unifying various second-position phenomena as instances of an interface condition on head extension. The condition requires a head to undergo at least two instances of merge within its phase. The core of the article explores properties of T0. It is shown that any merge (external or internal, merge of a head or a phrase) yields a well-formed structure. Since it does not matter to the requirement what category merges to T0, the condition must be stated as a general requirement on what category may be the root.
