It is practice, not law
European etiquette is not legislation and there is no authority that grants it. Nothing in it can be broken in the way a law is broken. What exists instead is a set of agreements a household keeps about how people are greeted, seated, fed, written to and sent home, and the standard is whatever that house has decided to hold itself to.
This matters because the question behind most etiquette questions is really a question about permission. May I wear this? May I leave now? May I ask? A house that keeps a standard answers those questions in advance, so nobody has to guess in the doorway and nobody has to be corrected in public.
Where the practice actually lives
Etiquette lives in households, not in hotels. A restaurant teaches service; a house teaches conduct. The difference shows at the edges of an evening: who is introduced to whom, where the guest of honour sits, whether the letter that follows is written within the day or within the fortnight, and whether the household’s own staff are addressed by name.
For that reason the most useful thing a person can learn is not a universal rule but a method: arrive at the hour stated, read the card rather than the memory of another card, follow the host’s lead in the first twenty minutes, and write afterwards. A method survives a change of country. A rule does not.
What it is for
Every courtesy worth keeping does one of three things: it removes a decision from someone who would rather not make it, it gives a person a place in the room, or it hands somebody something they can use. Introductions give a place. A named thank-you gives a host something to keep. A stated arrival time removes a decision from a kitchen.
Practices that do none of these things tend to be the ones that harden into performance, and performance is where etiquette becomes unkind. A standard that humiliates somebody has stopped being a standard. The test is not whether the room was impressed, but whether the person who knew the least felt least exposed.
Where it genuinely varies
Practice varies more than the books admit. In London an answer within the day is the courtesy; in Geneva the same courtesy is silence kept about what was said at the table. In Tokyo the exchange of cards carries information a European introduction carries in speech. In Dubai the room is arranged around the host in a way a Paris salon does not arrange itself.
It also varies inside a country, between a family that has kept one habit for three generations and a family that married into it last year, and between a grandmother and her granddaughter. Any page that presents one nation’s practice as universal law is describing a preference, not a country.
How a house comes to hold one
A standard is not adopted whole. It is written down, practised badly, argued about, amended, and written down again. The households that keep one well are almost never the ones that began with the most elaborate book; they are the ones that wrote a short book and used it, and that kept every earlier wording so a child could read what the house stood for and why it changed its mind.