Legal Term: Inheritance to the Eldest Son?

[Carol]

Steve asked:
> > > ... a legal term for the  process by which the Eldest son
> >  always inherits everything.
> > > 
> > > Can someone tell me what that term is?

Shaun responded: 
> > Are you thinking of primogeniture?

Steve again:
> Right, that it.
> 
> Thanks Shaun.
> 
Carol notes:

In the case of 12 GP, though, there's an additional provision, an entail that made it inheritable by the next male with the last name Black in the absence of sons. In the absence of any male heir, it would have gone to the oldest relative in the Black line, Bellatrix, if it weren't for Sirius's will. I'm not a lawyer, but I suspect that it Bellatrix or either of her sisters had been male, he could not have overridden the entail.

Carol, whose idea of entails comes from Jane Austen, not from any knowledge of estate law!