h21

H21, often written as h21, is an alphanumeric designation used in multiple, largely unrelated contexts rather than a single topic. The most documented use is in aviation, where the H-21 refers to the Piasecki H-21 Shawnee, a large tandem-rotor helicopter developed in the early 1950s for heavy transport, cargo lifting, and search-and-rescue missions. The aircraft demonstrated the viability of the tandem-rotor configuration and saw service with the United States military and allied operators, contributing to mid-20th-century advances in helicopter lift capability. It eventually exited front-line service as newer designs supplanted it.

Beyond aviation, h21 appears as a designation in transportation networks and infrastructure. In this context, it is used as a route, line, or service identifier within local or regional systems. Short alphanumeric codes such as h21 are common in transportation planning, so their meanings are tied to specific maps, timetables, and local conventions rather than to a universal standard.

In other fields, h21 may surface as a model number, product code, or internal designation within consumer electronics, automotive parts, software, or research projects. Because h21 lacks a universal, context-free meaning, the exact interpretation depends on the domain and accompanying documentation. For readers seeking a precise definition, it is important to specify the field or provide additional context to identify the intended reference.