> To take this further out in the weeds, one wonders who the Princes of
> Lichtenstein, Monoco, etc., are loyal subjects of, these days...

In both cases the princes are explicitly considered neutral fief
holders by reason of extinction, and conquest, respectively.

Traditionally a fief was established through a grant from the Feudal
Overlord, usually the King, or a ranking Vassal of his, with his
approval.

This was done independent of any titles and other priviledges (which
is why Feudal Baronies and Manors survive in the British Isles, where
they are now by and large treated as property titles and/or licences).
Typically someone of noble birth performed a service for a ruler, who
then rewarded him with what amounts to a tax collecter position, by
granting him lands, with the onus to pay a specific amount of tax (in
goods or metal) on an annual basis.

Now, the service performed most often consisted of military service,
or of bringing lands into the King's domain. It was a pretty good
system for all involved, as Barons began conquering their neighbours -
whose lands became automatically part of the realm and were 'granted'
back to the conquerors - who could be fairly certain that there would
be no reprisals from anyone, as their feudal lord, the king, could be
counted on defending them against aggression (in theory).

Over time the positions of tax collectors became heriditary and the
office of Count (go and count the revenue and lay taxes on the lands
at... ranging from... to...) became a title in it's own right. As
counties grew and some became more prosperous and influental, the
feudal lord had an interest in strengthening his influence and
control, while also protecting the lands themselves against incursions
from neighbours. To do that, he sent his brothers (Princes), to take
control of neighbouring counties, while leaving the ruling counts in
charge of their own fiefs.

As people didn't get that old in those days (especially the nobility),
kings often ran out of brothers before all borders were secured under
the rule of family members, so trusted allies were used when needed.
These, not being of Royal blood, couldn't style themselve princes, had
to outrank counts, and as they usually were retired ranking officers
and generals, became known as Earls.

Again, over time, Sons of Royal Princes, not technically royal
themselves anymore (thus the distinctions between Prince, royal
Prince, and Prince Royal) inherited the "Principalities"of their
fathers, swearing vassalage to the King on their ascension.
The same went for the Earls.

If however, a kingdom was partially conquered but some distant
hold-outs did not swear fealty to the conquerors, and were too strong
(or inconsequential) to fall to an easy conquest themselves, these
fiefs, not being subject to a King, became neutral by reason of
extinction - the previous kingdom was gone, the ruler of the fief
subject to no-one. And as only the church could crown heads, there was
a tendency of non-royal rulers to style themselves Marchois, or later,
Herzog, corresponding to the English Duke (not to be confused with
Russian Grand Dukes, Austrian Arch Dukes, etc.).

If they had royal pedigree, they kept (or took) the title of Prince,
which suggested that on resurrection of the kingdom they would have
first claim on succession.

So, to answer your question, The Prince of Monaco is of Royal
Burgundian blood, but would be the vassal of an extinct Southern
French king, which puts him beyond the reach of a French king.

The Prince of Liechtenstein is Vassal to the Holy Roman Empire which
went extinct when the German Reich was stablished and the assumption
of the title by the Austrian Emperor was disputed and ultimately
voided.

The Grand Duke of Luxembourg is similarly a Vassal of the Holy Roman
Reich, in theory, but would also have a claim to become Emperor of
both, France and the Reich, which is why everyone left them alone.

The King of Norway used to be a Vassal of the King of Sweden, and the
Queen of the Netherlands is technically subject to a 'familial union'
would make her subject of the Queen of England (whose forebearer
however rescinded that right when marrying into the house of
Hannover).

The King of Belgium is technically a vassal to Spain, not by lineage
but by real, and by way of Austria. He is technically also an ursurper
who has no right to the throne, which should be occupied by a
Habsburgian instead.

So hey, it all makes sense ;o)

Cheers,
Robert.