Mos maiorum

The mos maiorum (; "ancestral custom" or "way of the ancestors," plural "mores", cf. English "mores"; "maiorum" is the genitive plural of "greater" or "elder") is the unwritten code from which the ancient Romans derived their social norms. It is the core concept of Roman traditionalism, distinguished from but in dynamic complement to written law. The "mos maiorum" was collectively the time-honoured principles, behavioural models, and social practices that affected private, political, and military life in ancient Rome.
Family and society.
The Roman family (the "familia", better translated as "household" than "family") was hierarchical, as was Roman society. These hierarchies were traditional and self-perpetuating, that is, they supported and were supported by the "mos maiorum". The "pater familias", or head of household, held absolute authority over his "familia", which was both an autonomous unit within society and a model for the social order, but he was expected to exercise this power with moderation and to act responsibly on behalf of his family. The risk and pressure of social censure if he failed to live up to expectations was also a form of "mos".
The distinctive social relationship of ancient Rome was that between patron "(patronus)" and client "(cliens)". Although the obligations of this relationship were mutual, they were also hierarchical. The relationship was not a unit, but a network "(clientela)", as a "patronus" might himself be obligated to someone of higher status or greater power, and a "cliens" might have more than one patron, whose interests might come into conflict. If the "familia" was the discrete unit underlying society, these interlocking networks countered that autonomy and created the bonds that made a complex society possible. Although one of the major spheres of activity within patron-client relations was the law courts, patronage was not itself a legal contract; the pressures to uphold one's obligations were moral, founded on the quality of "fides", "trust" (see Values below), and the "mos". Patronage served as a model when conquerors or governors abroad established personal ties as patron to whole communities, ties which then might be perpetuated as a family obligation. In this sense, "mos" becomes less a matter of unchanging tradition than precedent.
Tradition and evolution.
Roman conservatism finds succinct expression in an edict of the censors from 92 BC, as preserved by the 2nd-century historian Suetonius: "All new that is done contrary to the usage and customs of our ancestors, seems not to be right." However, because the "mos maiorum" was a matter of custom, not written law, the complex norms that it embodied evolved over time. The ability to preserve a strongly-centralised sense of identity while it adapted to changing circumstances permitted the expansionism that took Rome from city-state to world power. The preservation of the "mos maiorum" depended on consensus and moderation among the ruling elite whose competition for power and status threatened it.
Democratic politics, driven by the charismatic appeal of individuals "(populares)" to the Roman people "(populus)", potentially undermined the conservative principle of the "mos". Because the higher magistracies and priesthoods were originally the prerogative of the patricians, the efforts of plebeians "(the plebs)" for access could be cast as a threat to tradition (see Conflict of the Orders). Reform was accomplished by legislation, and written law replaced consensus. When plebeians gained admission to nearly all the highest offices, except for a few arcane priesthoods, the interests of plebeian families who ascended to the elite began to align with those of the patricians, creating Rome's "nobiles", an elite social status of nebulous definition during the Republic. The "plebs" and their support of popular politicians continued as a threat to the "mos" and elite consensus into the late Republic, as noted in the rhetoric of Cicero.
During the transition to the Christian Empire, Quintus Aurelius Symmachus argued that Rome's continued prosperity and stability depended on preserving the "mos maiorum", and the early Christian poet Prudentius dismissed the blind adherence to tradition as "the superstition of old grandpas" "(superstitio veterum avorum)" and inferior to the new revealed truth of Christianity.
After the final collapse of the Western Roman Empire in 476 AD and ascension of the various Barbarian kingdoms, the old Roman mores were then either superseded by or synthesized with the traditions of the Germanic elite and subsequent feudal values.
Values.
Traditional Roman values were essential to the "mos maiorum":