The Spanish monarch acted as king (or with the corresponding title) according to the political constitution of each kingdom, state, or lordship,[3] and thus, their formal power varied from one territory to another.

The Monarchy included the Crown of Castile — with Granada, Navarre and the kingdoms of the Indies — and Aragon — with Sicily, Naples, Sardinia, and the State of the Presidi —, Portugal and its overseas territories between 1580 and 1640, the territories of the Burgundian Circle except between 1598–1621 — Franche-Comté, the Netherlands, as well as Charolais —, the Duchy of Milan, the Marquisate of Finale, the Spanish East Indies, and Spanish Africa.

As Francisco de Quevedo noted in España defendida, a work published in 1609, "properly, Spain is composed of three crowns: Castile, Aragon, and Portugal."

Regarding its structure, the Hispanic Monarchy was a composite monarchy where the "Kingdoms, States, and Lordships" that comprised it were united according to the formula aeque principaliter (or 'differentiated union'),[10] "under which the constituent kingdoms continued after their union being treated as distinct entities, so that they retained their own laws, charters, and privileges.

[11] Despite the respect and jurisdictional autonomy, there existed a common policy or directive that had to be obeyed, embodied by diplomacy and defense, with the Crown of Castile occupying the central and preeminent position over the others.