Complex sentence: Historically, the spelling of the county town and the county were not standardised, by the 18th century the names "County of Dunbarton" and "County of Dumbarton" were used interchangeably.
Split into simpler sentences: Historically, the spelling of the county town and the county were not standardised. By the 18th century the names "County of Dunbarton" and "County of Dumbarton" were used interchangeably.