When in Rome, do as the Romans do

1. When visiting a foreign land, follow the customs of those who live in it. It can also mean that when you are in an unfamiliar situation, you should follow the lead of those who know the ropes.

Idioms and Phrases

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.