when in Rome do as the Romans do

Cf. [St. Ambrose, quoted in St. Augustine Letters xxxvi. 32] (Migne), quando hic sum, non ieiuno sabbato; quando Romae sum, ieiuno sabbato, when I am here [i.e. Milan], I do not fast on Saturday; when I am in Rome, I fast on Saturday; [1660 quoted in verse form in Jeremy Taylor, Ductor Dubitantium (1851) I. i. 5. 5] cum fueris Romae, Romano vivito more, cum fueris alibi, vivito sicut ibi, when you're in Rome, then live in Roman fashion; when you're elsewhere, then live as there they live.

Whan tho herd hat Rome Do so of ther the dome [when you are at Rome do as they do there].
[c 1475 in Modern Philology (1940) XXXVIII. 122]

That which is commonly in euery mans mouth in England Whan you art at Rome, do as they do at Rome.
[1552 R. Taverner tr. Erasmus' Adages (ed. 3) 51^V]

My advice to you is among the Romans, do as the romans do.
[1766 in L. H. Butterfield et al. Adams Family Correspondence (1963) I. 55]

‘Do at Rome as the Romans do,’ is the essence of all politeness.
[1836 E. Howard Rattlin the Reefer I. xxii.]

‘I thought the English never bothered about protocol?’ ‘When in Rome, however, we do as the Romans do.’
[1960 N. Mitford Don't tell Alfred viii.]

One woman stationed there [Saudi Arabia] who purports to be comfortable with the rules said, ‘When in Rome, do as the Romans do.’ But how far does that go? To feeding the lions?
[2001 Washington Post 8 Dec. A25]

Related to: circumstances; conduct

Bibliography of major proverb collections and works cited from modern editions is available here.

Follow local custom, as in Kate said they'd all be wearing shorts or blue jeans to the outdoor wedding, so when in Rome--we'll do the same. This advice allegedly was Saint Ambrose's answer to Saint Augustine when asked whether they should fast on Saturday as Romans did, or not, as in Milan. It appeared in English by about 1530 and remains so well known that it is often shortened, as in the example.