The done thing (9)
(Other definitions for de rigueur that I've seen before include "Required by etiquette, as in France" , "Proper" , "Required by custom or etiquette" , "necessary" , "Obligatory" .)
(Other definitions for etiquette that I've seen before include "Rules of correct behaviour in society" , "Code of polite behaviour" , "form" , "Rules governing socially acceptable behaviour" , "Rules of correct social behaviour" .)