This paper focuses on auxiliary selection, which is the alternation between the auxiliaries BE and HAVE in the periphrastic perfect construction. This alternation mainly depends either on the argument structure (as in Italian), or on the person feature of the subject (as in many Southern Italian dialects). Crucially, the features of the arguments play a role in the distribution of the perfect auxiliaries even in argument-structure-based systems (for instance, cf. Italian reflexive clauses). On the basis of this observation, it is argued that auxiliary selection is the result of agreement for the person feature in both types of languages. The two-fold dependency on the argument structure and on the features of the arguments, required for languages such as Italian, leads to the formulation of Nested Agree, a principle that restricts the search domain of ordered Agree operations. The present analysis accounts for the whole distribution of the perfect auxiliaries in Standard Italian. In addition, it also provides an unified view of argument-structure-based systems and of person-driven systems of auxiliary selection.