1. Introductory remark The object of this diachronic research is the concept of habitual residence: its content and role. 2 Despite its absence in Roman law, this system contained a tiny collection of related concepts: citizenship and domicile. Domicile was especially developed conceptually during the republican and imperial époque. Roman law encompassed a factual and legal concept of domicile. Domicile was the reason for attachment of an individual to a particular urban community. Contrary to this, the concept of nationality made its way in national and private international law in the 19th century, in the era of building nation states. The concept of habitual residence was brought into international conventions only in the late 20th century. From the very beginning the concept retained its factual substance and unique conceptual role. Yet its role conditioned the content of the concept. The concept of habitual residence has been continuously favoured in private international law, mainly because of its descriptive and factual substance. The descriptive content of the concept facilitates its interpretation. This was one of the reasons for its transfer into Community law from the very beginning. In European Union law the concept was specified by the means of an ostensive and descriptive definition in the field of social and tax law and staff regulations. On the other hand, in the field of family law, the factual concept of habitual residence has played a unique role partially conditioned by its content. 2. Conceptual relationship between citizenship and domicile in Roman law The law of the tribunes M. Plautius Silvanus and C. Papirius Carbonus ( lex Plautia Papiria ) is invoked by Cicero in his defence of the poet Archius. 3 This law was enacted in the time of the Social War ( bellum sociale ) 89 B. C. Lex extended citizenship to Italian cities that were war allies of Rome. This law, unlike Lex Julia de Civitae Latinis Danda , comprised a wider scope of personal application. Not only cities but individuals as well were granted citizenship. According to Cicero, the law prescribed the conditions for acquiring citizenship as follows: Data est civitas Silvani lege et Carbonis: ‘Si qui foederatis civitatibus ascripti fuissent; si tum, cum lex ferebatur, in Italia domicilium habuissent; et si sexaginta diebus apud praetorem essent professi.’ Cum hic domicilium Romae multos iam annos haberet, professus est apud praetorem Q. Metellum familiarissimum suum. 2 ‚Substance‘ is synonymous with ‚meaning‘. 3 CICERO, Marcus Tullius. Řeč za básníka Archiu . Praha: F. Tempský, 1894, FRÝDEK, Martin. Jak se z cizince mohl stát našinec – římskoprávní úprava vzniku státního občanství a proces proti básníkovi Archiovi. JÍLEK, Dalibor and POŘÍZEK, Pavel. Pobyt cizinců: vybrané právní problémy . Brno: Wolters Kluwer, 2015, pp. 243-250.