Spain: Maritime Provinces

The origin of the Maritime Provinces goes back to King Phillip III's Royal Order of 5 October 1607 that first divided Spain into the three Maritime Provinces of Ferrol, Cádiz and Cartagena, to order the different sea ports and their ships. In 1845, a Royal Decree established 36 Maritime Provinces, six overseas Provinces included. The Maritime Provinces went through many changes, by creation of new ones, merging of former ones, or suppression of former ones.

José Carlos Alegría, 5 November 1999

The Royal Decree issued in 1845 assigned a flag for each province, usually known as matricular or register flag, and ordered the merchant ships to fly the flag of their harbour's maritime province at masthead. That way it was possible to identify not only the ship's nation, but her local provenance, where it was registered.
The difference between the European provinces and those from overseas territories are that the first were rectangular flags, while the latter were swallow-tailed.
In 1923, the Register of Ships published the first colour chart with the flags of the Maritime Provinces.