Royal fifth

The royal fifth (quinto real or quinto del rey in Spanish and Portuguese) is an old royal tax that 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 instituted in Medieval Muslim states, Christian Iberian kingdoms and their overseas colonial empires during the age of exploration.

The 'royal fifth' has a dual origin. In Christian kingdoms, it 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.) within his jurisdiction. Consequently, private individuals who extracted these natural resources owed compensation to their original 'owner' (the monarch).

The specification of the 20% tax rate on war booty, stems from the practice in Islamic states. It was institutionalized from the start of the Islamic conquest, with the rate set down in the Qur'an, in Sura VIII (Al-Anfal), verse 41:

In practice, the share of the fifth reserved to the Prophet's family lapsed after Muhammad's death. The early Rashidun Caliphs, notably Caliph Omar, set down regulatory guidelines for what could and could not be regarded as war spoils, and assigned the fifth for welfare distribution. The 'fifth' eventually became an important source of financing for the Caliphal administration and army. Schools of Islamic law were divided on whether the fifth extended to treasure troves and mining. Some schools (notably, the Hanafite), regarded treasure and mines as 'spoils' and thus subject to the fifth, while others (notably the Shafi'ite and Hanbalite) regarded them as subject only to the conventional rates, e.g. zakat.

...
Wikipedia