The previous hierarchical system of codes has been replaced as from January 2011 by a nine-character code for all types of geography, in which there is no relation between the code for a lower-tier area and the corresponding parent area. The older coding system has now been phased out.
Electoral wards (called electoral divisions in Wales). These are the areas defined for the election of local councillors, but are also widely used for presenting statistics at a smaller scale than the whole district.
Census output areas (OA). These are the smallest unit for which census data are published - they were initially generated to support publication of 2001 Census outputs and contain at least 40 households and 100 persons, the target size being 125 households. They were built up from postcode blocks after the census data were available, with the intention of standardising population sizes, geographical shape and social homogeneity (in terms of dwelling types and housing tenure). The OAs generated in 2001 were retained as far as possible for the publication of outputs from the 2011 Census (less than 3% were changed). Before 2001, census data were published for larger Enumeration Districts (ED) which were delineated before the census was conducted and were the organisational units for census data collection.