Royal fifth

The royal fifth (Spanish and Portuguese: quinto real / quinto del rey) is a historical royal tax which reserves to the monarch 20% of all precious metals and other commodities (including slaves) acquired by his subjects as war loot, found as treasure or extracted by mining.

The 'royal fifth' was first instituted in medieval Muslim states from interpretations of Qur'an, though the extent to which it applied was debated between schools of Islam.

[1] The medieval Taifa kingdoms of al-Andalus embraced the Hanafi argument and institutionalized the fifth on war spoils, treasure troves and mining.

[2] In Christian kingdoms, the royal fifth partly comes from the medieval legal conception of seigneural or regalian rights over the natural patrimony, which assigned to the monarch or feudal overlord original property rights over all unclaimed, undiscovered and undeveloped natural resources (e.g. precious metals in the subsoil, salt in the rock, virgin forests, fish in the sea, etc.)

In Spain, the quinto real on mining of precious metals was codified by the edict of February 1504 and (with occasional exceptional grants) remained in force through all the Spanish empire until the 18th century.

The Spanish government had a monopoly of mercury production, through its mines at Almadén in Spain and at Huancavelica in Peru.