This paper examines the distribution of subject agreement suspension, i.e. the phenomenon whereby the verb does not agree in gender and number with the subject and appears in a default form, in a corpus of Old Tuscan texts and in three different varieties of Modern Tuscan. The main aim of the study is to identify the formal and functional factors governing the appearance of subject agreement suspension and its development over time. It will be argued that subject agreement suspension in main and subordinate clauses is not a unitary phenomenon. It will be shown that subject agreement suspension with post-verbal subjects in main clauses is associated with focal subjects, both in sentence- and argument-focus constructions. Subject agreement suspension in relative and complement clauses, not attested in Old Tuscan, is determined by agreement of the verb in the subordinate clause with the invariable relativiser/complementiser che, which bears no features and therefore triggers default agreement on the verb.