The lordship of Piedmont, later the principality of Piedmont (Italian: Piemonte), was originally an appanage of the County of Savoy, and as such its lords were members of the Achaea branch of the House of Savoy.
The title was inherited by the elder branch of the dynasty in 1418, at about which time Savoy was elevated to ducal status and Piedmont to princely status.
When the House of Savoy was given the Kingdom of Sardinia, the Savoyards used the style of Prince of Piedmont (Italian: Principe di Piemonte) for their heir apparent.
The usage was retained when Victor Emmanuel II became King of Italy, "Prince of Piedmont" becoming roughly equivalent to the British "Prince of Wales", the title bestowed to the Crown prince.
When the House of Savoy became the ruling dynasty of Italy in 1861, they continued to use the title of Prince of Piedmont for the heir apparent but also began alternating it with a new title, the Prince of Naples.