Back in the early 1900's when motor vehicles first graced the roads, motorised vehicles were not registered with car registration plates. After all, who would have thought it would take off. Initially a motor car had to be led by a man with a flag to wave pedestrians out of the way. The ad hoc arrangements that were initially put in place to register vehicles soon led to conflicts as to who owned what and so the authorities then decided that each vehicle needed to have some sort of Identification.
Number Plates By UK AreaIt soon became apparent that the system of car number plates needed to be a national scheme split up into hundreds of localised areas. Each of these areas or authorities had their own individual identifiers. 'A' was London, 'AA' Hampshire, 'AB' Worcestershire etc. This meant that each authority could issue 1000 car registration plates as the numbers went up to 9999.
When they were all issued, they simply reversed the plate and put the numbers first. This meant another 1000 cars could be registered. Once the reversed combinations were all used up the local authorities simply added a letter at the front of the plate so AA became AAA.