Original: Over its existence, the administrative county lost territory, both to the existing county boroughs, and also due to the municipal borough of West Hartlepool becoming a county borough in 1902 and Darlington in 1915.
Simplified: Over its existence, the administrative county lost territory, both to the existing county boroughs, and also due to the creation of county boroughs at West Hartlepool in 1902 and Darlington in 1915.