The first to hold the title was George Douglas, a younger son of the first Marquess of Douglas, who was created Earl of Dumbarton in 1675.
A Scottish nobleman and soldier, George was given the title by Charles II in recognition of his military service.
When James VII of Scots (James II of England) came to the throne in 1685, the earl was made military commander in Scotland, and it is thought that when the King was deposed, George accompanied him to his court in exile in France.
The title became extinct when the second earl died in 1749, meaning it has not been used in more than 260 years.
It means that Prince Harry becomes the third to hold the Scottish title, and on marriage, Meghan will become Countess of Dumbarton.
Dumbarton is on the north bank of the River Clyde, to the west of Glasgow.
The town was founded in the fifth century and was once the capital of the ancient Kingdom of Strathclyde.
It is the site of a volcanic rock which is home to Dumbarton Castle. During its long history, the castle has been both a royal residence and a fortress.
Dumbarton was a Royal Burgh between 1222 and 1975.