Categories (introduced to Wikipedia in 2004 and since available in all sites using MediaWiki software) are an excellent way of linking related pages and automatically listing them in alphabetical order. Spin-off benefits include the ability to see if there are gaps in a subject or inconsistencies in naming or grouping. Every category except should be a subcategory of at least one category, but will often be suitable for two or more; for example, "" could be in both "" and "". Similarly, every article should be in at least one category but will very often be suitable for two or more.