21A

21a is an alphanumeric designation used in various systems to label items that share a base number with a letter suffix. The letter typically signals a variant, subdivision, or alternate form of the item identified by the base number 21. Because there is no single entity named “21a,” its meaning depends on the context in which it is used.

In transportation, 21a commonly denotes a spur, connector, or alternate alignment associated with route 21. In public transit, a bus or rail line may be labeled 21A to indicate a branch service that operates in addition to the main 21 line. The specific routing and service patterns of such labels vary by country and municipality.

In legal and governmental documents, 21a can designate subdivisions, amendments, or specific provisions within larger statutes, regulations, or forms. This usage helps differentiate closely related clauses or sections and is common in court citations, administrative rules, and legislative drafting.

In catalogs, product lines, and media collections, 21A can designate a variant edition, version, or release. For example, a catalog might list 21A as a revised or regional edition of a base item labeled 21, or as a distinct model in a series.

Other uses include codes in academic forms, datasets, or organizational inventories where alphanumeric schemes help manage multiple related items. Because 21a spans many domains, locating a precise meaning requires accompanying context.

See also: other alphanumeric suffixes such as 21, 21b, A21.