Etydhv

I just don't understand the concept of principalities, why wouldn't you just call yourself king?

The job description appears to be exactly the same.

When England was ruled by the Saxons we had a bunch of kings, who slowly all got defeated and the winner declared the old king a Duke (or worse).

Is it when an empire got to big to manage the King set his sons off to manage chunks and over time they became more autonomous?

I tried to google but all I could find really were about the bible and demons. One article about wales looked really promising but it just seemed to be an extract about a book.

I looked at the proposed duplicate How were nobles 'promoted' in aristocracies?.
A better Example is Queen Victoria deciding to become Empress of all India after her daughter became Empress of Germany. (I know it is politically more complicated than that really)

This thread doesn't answer my question as principalities weren't under a king who had given them their title and who they owed fealty to. Principalities are top of their own pile. Somebody has answered my question with the Russian Tsar example

I have just read the other suggested link Why is the heir to the British throne called "Prince of Wales"?
and the fact that prince actually means leader is very interesting.

If you are a prince you may call yourself whatever you want. And you can try to impose the usage of the title on your subjects, if you really care. But this is not the main point. The main point is the recognition by your neighboring princes and kings.

A good case to illustrate this is the story of Russia. It used to be the Grand Duchy of Moscow, or simply Moscovia. After the conquest of several neighbors, including one or two who called himself Tsars, the prince
decided to call himself Tsar. (Which means somewhat more than a king, rather an emperor). Those neighbors not yet conquered refused. But the Russian prince insisted. This created a lot of diplomatic problems and even wars with the principal neighbor (Polish-Lithuanian Commonwealth). Indeed, several times the problem of properly addressing the prince was a cause of war.

The situation changed as a result of the Great Northern war in which Moscovia defeated Sweden, though the main result of this war was conquest of what is called nowadays Eastern Ukraine and converting the Polish-Lithuanian Commonwealth into
a satellite (to be completely conquered few decades later). The result of this war
was an international recognition of the "Emperor of all Russias". The struggle for this title lasted almost 200 years. (Some historians say that this Northern war was the most devastating war in Russian history, in terms of relative loss of population).

Another source of the higher title is some higher authority. For example Prince Danylo of Galicia was crowned the King of Galicia by the Roman Pope,
in exchange for the recognition of the Pope authority.

As an extreme example I can mention an adventurist Jørgen Jørgensen who in 1809 declared himself the "King of Iceland". Apparently the Icelanders had nothing against it, so he "ruled" for two months, until a British ship arrived and he was arrested. Source: https://guidetoiceland.is/history-culture/the-top-7-most-infamous-icelanders-of-history. Moral: you need at least some degree of international recognition to become a king:-)

Expanding on Alex' answer, there is the example of the Hohernzollern dynasty. They were the prince-electors of Brandenburg, but from 1701 to 1772 they called themselves kings in Prussia. They did not call themselves king of Prussia yet because Brandenburg was part of the HRE and their kingship was only seen as valid outside the HRE -- in Prussia, not in Brandenburg.

After some generations they got the other German princes and kings to accept their kingship without the qualifier.

Sometimes sovereigns had to downgrade their title. We often think of Franz Josef as Emperor of Austria-Hungary after the creation of the dual monarchy (with Hungary having won autonomy from Austria in 1866-1867) since he previously had been Emperor of Austria. But in fact he was Emperor of Austria and King of Hungary, complicating many things...e.g., the joint army was called "the imperial and royal army" as the Hungarians insisted it not be called "the imperial army".

Several people did just that: have control over a territory, declare themselves to be $insert–title.

Odoacer declared himself to rex Italiae, king of Italy

Fall and death

As Odoacer's position improved, Zeno, the Eastern Emperor, increasingly saw him as a rival. According to John of Antioch, Odoacer exchanged messages with Illus, who had been in revolt against Zeno since 484.

Napoleon

To expand his power, Napoleon used these assassination plots to justify the creation of an imperial system based on the Roman model. He believed that a Bourbon restoration would be more difficult if his family's succession was entrenched in the constitution.

Wilhelm I

William I, German Emperor

Instead of Emperor of Germany! The feelings of other princes in Germany were an obstacle, and other European rulers or countries as well.

They all have in common that it was difficult for them to not come across as parvenus. Upstarts make for jealous peers and neighbours, all keen to not fall behind or loose their standing.

That is, it all depends whether it is perceived as something real.

The Emperor of the United States began his rule with well formed reason and conviction, after all:

By 1859, Norton had become completely discontented with what he considered the inadequacies of the legal and political structures of the United States. On September 17, 1859, he took matters into his own hands and distributed letters to the various newspapers in the city, proclaiming himself "Emperor of these United States".

If one can pull it off, fine. It looked for a while that Napoleon could do it. Wilhelm was also in a position to hold on until his death.

It's difficult enough to subjugate your own subjects, many kings and Roman or even Holy Roman Emperors knew the lyrics of that song by heart. But during several epochs, usurping the wrong title could mean imminent attack by rivals. Rivals who showed them who was king.