Caernarfonshire was created under the Statute of Rhuddlan in 1284 combining the cantrefs of Arfon, Arllechwedd, Llyn and the commotes of Eifionydd and Creuddyn.
Under the Local Government Act of 1888 the administrative county of Caernarfonshire was formed with boundaries identical to the geographic county and used for the purpose of local government between 1889 and 1974. Under the Local Government Act of 1972, the administrative county of Caernarfonshire was abolished in 1974 and eight preserved counties were created for local governance in Wales. While the government maintained that the Local Government Act of 1972 did not affect the historic county itself, Royal Mail encouraged everyone to use the new preserved counties on addresses and the media started using the new administrative areas as actual counties.
Since the local government reorganisation in 1996, Royal Mail have advised against the use of postal counties and will be deleting postal counties completely from their database between 2013 and 2016.