Claims to a crown
There are five ways in which a person may lay claim to a crown. The below ordering is based on strength of case and possession.

Right of Conquest: If one overthrows the monarch, taking the crown and kingdom by force, and holds them, then one is monarch.  Usurpation and deposing of the monarch fall into this category.
Presumption: In the absence of a monarch, if one lays claim the crown and kingdom without resistance and can hold them, then one is monarch.
Right of Royal Succession: When the monarch dies, should the law prescribe the succession of the crown and kingdom, and one is numbered first in that succession, then one is monarch, so long as no other person usurps the crown.
Right of Nomination: Should the monarch die leaving one as the designated heir, in the absence of law prescribing succession of the crown and kingdom, then one is monarch, so long as no other person usurps the crown and one can quell all other claimants.
Right of Kinship: Should the monarch die leaving no designated heir, and in the absence of law prescribing succession of the crown and kingdom, and one is the closest relative by kinship to the deceased monarch, then one is monarch, so long as no other person usurps the crown and one can quell all other claimants.