Principality of Piedmont in the context of "Principality of Achaea"

The term Kingdom of Sardinia denotes the Savoyard state from 1720 to 1861. From 1720 to 1847, only the island of Sardinia proper was part of the Kingdom of Sardinia, while the other mainland possessions (principally the Duchy of Savoy, Principality of Piedmont, County of Nice, Duchy of Genoa, and others) were held by the House of Savoy in their own right, hence forming a composite monarchy and a personal union, which was formally referred to as the "States of His Majesty the King of Sardinia". This situation was changed by the Perfect Fusion act of 1847, which created a unitary kingdom. Regardless, historians often use "Sardinia" as a synecdoche to designate the whole Savoyard state from 1720. Due to the fact that Piedmont was the seat of power and prominent part of the entity, the state is also referred to as Sardinia–Piedmont or Piedmont–Sardinia, and sometimes erroneously as the Kingdom of Piedmont.

Before becoming a possession of the House of Savoy, the medieval Kingdom of Sardinia had been part of the Crown of Aragon and then of the burgeoning Spanish Empire. With the Treaty of The Hague (1720), the island of Sardinia and its title of kingdom were ceded by the Habsburg and Bourbon claimants to the Spanish throne to the Duke of Savoy, Victor Amadeus II. The Savoyards united it with their historical possessions on the Italian peninsula, and the kingdom came to be progressively identified with the peninsular states, which included, besides Savoy and Aosta, dynastic possessions like the Principality of Piedmont and the County of Nice, over both of which the Savoyards had been exercising their control since the 13th century and 1388, respectively.

The Perfect Fusion (Italian: Fusione perfetta) was the 1847 act of the Savoyard King Charles Albert of Sardinia which abolished the administrative differences between the mainland states (Savoy and Piedmont) and the island of Sardinia within the Kingdom of Sardinia, in a fashion similar to the Nueva Planta decrees between the Crown of Castile and the realms of the Crown of Aragon between 1707 and 1716 and the Acts of Union between Great Britain and Ireland in 1800.

The once-Aragonese Kingdom of Sardinia had become a possession of the House of Savoy in 1720, and it had continued to be ruled as during the era of the Spanish Empire.

The Duchy of Aosta, originally the County of Aosta (French: Duché d'Aoste), was a realm ruled by the House of Savoy from the early 11th century until the late 18th, when its independent institutions were aligned with those of the Principality of Piedmont. The title "Duke of Aosta" continued to be used by the second sons of the Savoyard monarch. The land of the duchy is today a part of Italy.

The county of Aosta was originally ruled by the bishops of Aosta in the 10th and early 11th centuries. Upon the death of Bishop Anselm in 1026, however, Conrad the Salic ensured that the secular powers of the important Alpine territory passed to the bishop's brother-in-law, his ally Humbert the White-handed, rather than remaining tied to the diocese, which fell to Anselm's unfriendly nephew Burchard. Humbert's son Odo then wed Adelaide, securing the March of Turin. The county was elevated to a duchy by Frederick Barbarossa.

Archaeological evidence of prehistoric human settlement on the island of Sardinia is present in the form of nuraghes and other prehistoric monuments, which dot the land. The recorded history of Sardinia begins with its contacts with the various people who sought to dominate western Mediterranean trade in classical antiquity: Phoenicians, Punics and Romans. Initially under the political and economic alliance with the Phoenician cities, it was partly conquered by Carthage in the late 6th century BC and then entirely by Rome after the First Punic War (230 BC). The island was included for centuries in the Roman province of Sardinia and Corsica, which would be incorporated into the diocese of Italia suburbicaria in 3rd and 7th centuries.

In the Early Middle Ages, through the European barbarian movements, the waning of the Byzantine Empire influence in the western Mediterranean and the Saracen raids, the island fell out of the sphere of influence of any higher government; this led to the birth of four independent kingdoms called Judicates (Latin: Judicati; Sardinian: Judicados) in the 8th through 10th centuries. Falling under papal influence, Sardinia became the focus of the rivalry of Genoa, Pisa, and the Crown of Aragon, which eventually subsumed the island as the Kingdom of Sardinia in 1324. The Iberian Kingdom was to last until 1718, when it was ceded to the House of Savoy; from Piedmont, the Savoyards pursued a policy of expansion to the rest of the Italian peninsula, having their Kingdom of Sardinia be later renamed into "Kingdom of Italy" in 1861.