Which son of Charlemagne was known as 'Charles the Younger'?

Charlemagne’s son known as “Charles the Younger” was Charles. He was born around 772 or 773 to Charlemagne and Queen Hildegard, and became one of his father’s principal military deputies and political heirs.

Charles received a broad education typical of the Carolingian court, including the liberal arts, riding, hunting, and warfare. From 784 onward, he commanded forces during the Saxon Wars. In 789, Charlemagne entrusted him with rule over lands west of the Seine, corresponding roughly to the Duchy of Maine.

Charles was publicly elevated during the imperial coronation at Rome on Christmas Day 800, when Pope Leo III crowned Charlemagne emperor and anointed Charles as a king. Charlemagne’s 806 succession plan assigned him the largest share of the inheritance, including Francia and Saxony. However, Charles died in 811, before his father, so the empire ultimately passed to his younger brother Louis the Pious.