This variationist analysis investigates the development and spread of innit as an invariant tag in London English. The sociolinguistic distribution of innit in a socially stratified corpus of vernacular speech suggests that the form's emergence and spread were initiated and propelled system-internally through changes associated with grammaticalization. Frequency triggered phonetic reduction of isn't it to innit; loss of syntactic-semantic usage constraints and growing functional versatility enabled innit to seize the range of contexts and functions of grammatically-dependent tags (e.g. didn't you, weren't we), virtually ousting these from the system of negative-polarity interrogative tags. Examination of cross-linguistic data and comparisons with relevant pre- and non-contact varieties indicate multiple language contact and grammatical replication may have played an ancillary role. I flag some challenges of establishing contact effects in discourse-pragmatic change, and propose that the promotion of innit for invariant use was governed by its low salience and social indexicality of localness. (Innit, question tags, (Multicultural) London English, grammaticalization, language contact, grammatical replication)*