We is often used instead of I by monarchs.
If you look at the example on Wikipedia https://en.wikipedia.org/wiki/Royal_we
>Now, We, Edward, by the grace of God, King of the United Kingdom of Great Britain and Ireland and of the British Dominions beyond the Seas King, Defender of the Faith, Emperor of India, etc., etc., have arrived at the following decisions upon the questions in dispute, which have been referred to Our arbitration, …
The origin is because in public they do not speak as individuals but the representative a country, institution etc
Colloquially, (ie not precisely correct) the term is used when someone refers to themselves in the third person. Rather than “I would like to buy an apple” they use “We would like to buy an apple”. To poke fun at that usage, this can be referred to as using the “royal we”.
As others have mentioned, monarchs when they speak/write formally (British royalty perhaps since this is English), use the term “we” instead of “I” because they (presumably) speak for the nation or the court.
Traditionally, people in high office, such as monarchs, have often used “we” even when they’re only talking about themselves. This can be because they see themselves as representing a nation or organisation, or it can just be a way of sounding grandiose and signalling that they view others as beneath them. It’s quite common for people to be mocked or criticised for doing this, though in a lot of cases it’s unclear whether that was their intent, or whether it was a slip of the tongue or they were trying to indicate that they were speaking on behalf of some specific group of people. An infamous example was when Margaret Thatcher said “We have become a grandmother.”
There are also lots of situations where someone uses “we” in a different sense and people wrongly understand it to be the royal we. For example, many authors use “we” to mean “you and I”, and it’s common for people to use “we” to talk about a group of people without explicitly saying who they are talking about (“we went to see that band” = “my partner and I went to see that band”, “this thing happened at work and we were all stressed out” = “my colleagues and I were stressed out”).
The “royal we”, when used in casual conversation, really is a joke, meaning the person is really saying YOU, not WE.
If someone says “We should really clean the kitchen. And I mean the royal we!” That means they want you to do it.
It’s called “royal” because it’s how a king or other royalty might speak when they are giving a command. If the king says “we should have a feast this weekend” he certainly doesn’t mean HE is going to do anything to plan or prepare for a feast. He’s telling whoever he’s talking to that they need to arrange it.
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