This article is about Royal Navy and European ship pennant numbers. For the United States equivalent, see Hull classification symbol. For the Canadian equivalent, see Hull classification symbol (Canada).

In the Royal Navy and other navies of Europe and the Commonwealth of Nations, ships are identified by pennant number (an internationalisation of pendant number, which it was called before 1948). Historically, naval ships flew a flag that identified a flotilla or type of vessel. For example, the Royal Navy used a red burgee for torpedo boats and a pennant with an H for torpedo boat destroyers. Adding a number to the type-identifying flag uniquely identified each ship.