Local government reorganisation was first flagged up in a 1990 green paper put forward by then environment secretary Michael Heseltine.
The process was begun by the Local Government Commission, set up in 1992 by Parliament.
Wales and the Isle of Wight led the way on unitary status. Eight Welsh counties were replaced by 22 unitary authorities in 1995.
Cleveland and Humberside were replaced by 12 unitaries in 1996, the same year that York left North Yorkshire.
Berkshire disappeared this week, replaced by five of a total of 19 new councils. The class of 1998 includes Plymouth, Telford and the Wrekin, and Slough.
So far, five English counties have been abolished and 46 unitary authorities created.
In England, reorganisation has created an often confusing mixture of two-tier and unitary authorities. For example, the new Hampshire remains but the county has lost the port cities of Portsmouth and Southampton to new unitaries.