Wikipedia:Lists of lists

A List-of-lists article is a specialized type of stand-alone list article that is mainly or entirely a list of other stand-alone list articles. This essay provides guidance about the content and purpose of such lists, how to name them, how to place them in categories, and the difference between list-of-lists articles, disambiguation pages, and set index articles.

General
A list-of-lists article is an article that is primarily or entirely a list of other list articles. An article such as List of unsolved problems in mathematics, which contains a number of lists but does not give links to other list articles, may or may not be considered a list-of-lists article. However, a hybrid article such as Lists of Coptic church buildings, which contains lists and also links to other list articles like List of Coptic Orthodox churches in Australia, is considered a list-of-lists article.
 * A list-of-lists article may contain metadata, subheadings, tables, and extra information about each entry, and may contain references. Examples include Lists of volcanoes, Lists of mathematicians and Lists of law clerks of the Supreme Court of the United States.
 * Nonexistent lists should not be included in a list-of-lists article. That is, all the links in a list-of-lists article should be active (blue, not red).
 * A list-of-lists article should comply with the Stand-alone lists guidelines for content, format, and style.
 * The lead section will typically describe the type of thing the lists cover, with links to relevant articles, and the organization of the lists, as with Lists of Victoria Cross recipients.

Purpose
A list-of-lists article provides an index to a collection of related list articles, and helps readers navigate to the article they want. It may also provide information about the lists or the topic as a whole. Common purposes of a list of lists include: See Lists of companies for an example of a list-of-lists article that includes a wide variety of different types of list. A given company may be in several of these lists.
 * Provide an index to a large list that has been broken down into sub-lists. For example, Lists of airlines gives sub-lists of airlines by continent like List of airlines of Africa, which in turn gives lists for each country.
 * Provide an index to lists that present the same entries in different sequences. For example, Lists of countries by GDP per capita gives lists by nominal and PPP gross domestic product per capita.
 * Provide an index to lists that present selections of similar things. For example, Lists of earthquakes gives lists of the largest, costliest, deadliest, etc.

If the list-of-lists article does no more than show the same lists in the same alphabetical sequence as the corresponding list-of-lists category, it is questionable whether it has a valid purpose. A useful list-of-lists article will normally present the lists in a more structured form than is possible with a category and/or provide more information than just the names of the lists.

Since the purpose of a list-of-lists is primarily navigational, it may have few or no incoming links from other articles. It should not be tagged as Orphan. However, it is common to have a link to the list-of-lists in the "See also" section of each of the child lists, so readers can compare related lists.

Naming
The recommended, and by far most common, title format is "Lists of X". Some articles are at names of the form "List of X", though often this will be because the article began as a stand-alone list, which was later split due to size into multiple list articles, with the original article serving as an index (e.g. List of New York City Designated Landmarks in Manhattan, List of Latin phrases). Whichever form is chosen, the other should generally be created as a redirect.

Categorization
All List-of-lists articles should be categorized in Category:Lists of lists, or in a subcategory such as Category:Lists of entertainment lists‎ or Category:Lists of lists of people‎. These "type" categories are broad in scope and are mainly used for development. They should help an editor working on a new list-of-lists article who wants to use similar existing articles as blueprints.

List-of-lists articles should also have a corresponding "lists" category, often with the same name. For example, the article Lists of ports is categorized in Category:Lists of ports. All the lists in the article Lists of ports should also be categorized in Category:Lists of ports. The main list-of-lists article should be shown in the corresponding "lists" category with a cat main hatnote, e.g.

Sometimes a list-of-lists article is placed in additional categories. However, it is usually better to place these categories in the corresponding "lists" category, since they usually apply to all the lists in the list-of-lists. For example, Category:Lists of ports is in Category:Ports and harbours, which applies to all lists of ports, not just the list-of-lists of ports.

Template List of lists may be used to tag and categorize list-of-lists articles, equivalent to disambiguation and Set index article. It automatically populates the following tracking categories:
 * Category:Lists of lists with listcat specified: consider renaming the list-of-lists or the list category so they are the same
 * Category:Lists of lists with no matching category: consider creating a "lists" category for lists in this list-of-lists. If a matching category does not seem reasonable, consider whether the list-of-lists article should be converted to a disambiguation page.

Splitting long lists
Sometimes a long list is split into several smaller ones to conform to the Article size guideline. Thus  may be split into ,   and. See Naming conventions (long lists) for a discussion. There is usually no need to leave a list-of-lists index at the original title. Instead,  can redirect to , with the three sub-lists all in the same categories as the original long list. A reader searching for "list of foos" will see all the sub-lists early in the search results, and can go directly to the one they want. Hatnotes can support easy navigation between the sub-lists:
 * Lists of foos: A–D | E–M | N–Z

There are times when a large list is split into a small number of sub-lists leaving an index at the original title. Thus we could have, pointing to   and. The parent list may be useful as the target of links from other pages, although explicit links to the member lists may be more convenient to readers. A very small index like this may be better treated as a disambiguation page (see below). Check first that there are no inbound links to the index, or convert the inbound links to point to the sublists, e.g.

However, a small list-of-lists index is sometimes more useful than a disambiguation page, perhaps because it provides an introduction and explanation of the sub-lists, or perhaps because it transcludes selected sections from the sub-lists to provide a summary. In this case, rather than make a separate category for the index and its members, they may be placed at the front of a broader category. Thus  may be placed in   and   and    may be placed in.

A short list like  containing just two lists: List of cities in Morocco and List of cities in Western Sahara serves no valid function, since it will not be the target of links from other pages. It should be deleted.

Comparison with Set index articles and Disambiguation pages
Historically, list-of-lists pages have often been tagged as set index articles. However, there is consensus against this practice. A list-of-lists article should also never be classified as a disambiguation page. The main differences are: As a general rule, if a list meets the definition of a list-of-lists, it should not be treated as a set index article. A list-of-lists article is concerned with list articles, and a set index article is concerned with other types of article. However the distinction between a list-of-lists article and a disambiguation page can be fuzzy. Some possible tests:
 * A list-of-lists article is a list article that is mainly or entirely a list of other lists, which may or may not have similar names (e.g. Lists of mathematics topics). It may contain metadata and extra information about each entry, and may contain references, but should not contain redlinks.
 * A set index article is a list article about a set of items of a specific type that also share the same (or similar) name. A set index is meant to provide information as well as navigation, and can have metadata and extra information about each entry. It may contain red links to help editors create articles on notable entries, and may contain references.
 * A disambiguation page is a list of things, possibly of different types, that share the same (or similar) name. Its purpose is to take a reader quickly to the article they are looking for. It should generally not contain red links, should contain only the minimal information needed to distinguish the entries, and should not contain references.
 * Do other pages have relevant links to this page that have no obvious alternative target? If so, it is probably a valid list of lists. An example could be a large list that has been broken into sub-lists, leaving the main title as an index.
 * Are the entries in the list concerned with the same type of thing, or do they just have the same or similar names in English?
 * If a reader searching for information on a particular topic might find all the entries relevant, it is a valid list of lists.
 * If there is an article that describes the types of thing found in all the lists, it is a valid list of lists.
 * If the list article would seem incoherent when translated into a different language, it is a disambiguation page. For example, List of museums in Georgia contains List of museums in Georgia (country) and List of museums in Georgia (U.S. state). But in the Polish language, the country is Gruzja and the state is Georgia. Lumping them together makes no sense in Polish, although it is useful in English for the purpose of disambiguation.
 * Would the associated list-of-lists category provide a coherent grouping of related lists? If not, it is a disambiguation page.
 * Does the list contain a small number of lists that do not overlap in content and do not form the parts of an overall list? If so, it is probably a disambiguation page.

See Wikipedia talk:Disambiguation/Archive 52 for a more detailed discussion.