A postal code is a series of letters and/or digit

Numerical digit

A digit is a symbol used in combinations to represent numbers in positional numeral systems. The name "digit" comes from the fact that the 10 digits of the hands correspond to the 10 symbols of the common base 10 number system, i.e...

s appended to a postal address

Address (geography)

An address is a collection of information, presented in a mostly fixed format, used for describing the location of a building, apartment, or other structure or a plot of land, generally using political boundaries and street names as references, along with other identifiers such as house or...

 for the purpose of sorting mail

Mail

Mail, or post, is a system for transporting letters and other tangible objects: written documents, typically enclosed in envelopes, and also small packages are delivered to destinations around the world. Anything sent through the postal system is called mail or post.In principle, a postal service...

. Once postal codes were introduced, other applications became possible.

In February 2005, 117 of the 190 member countries of the Universal Postal Union

Universal Postal Union

The Universal Postal Union is an international organization that coordinates postal policies among member nations, in addition to the worldwide postal system. The UPU contains four bodies consisting of the Congress, the Council of Administration , the Postal Operations Council and the...

 had postal code systems. Countries that do not have national systems include Ireland

Republic of Ireland

Ireland , described as the Republic of Ireland , is a sovereign state in Europe occupying approximately five-sixths of the island of the same name. Its capital is Dublin. Ireland, which had a population of 4.58 million in 2011, is a constitutional republic governed as a parliamentary democracy,...

 and Panama

Panama

Panama , officially the Republic of Panama , is the southernmost country of Central America. Situated on the isthmus connecting North and South America, it is bordered by Costa Rica to the northwest, Colombia to the southeast, the Caribbean Sea to the north and the Pacific Ocean to the south. The...

. Although Hong Kong

Hong Kong

Hong Kong is one of two Special Administrative Regions of the People's Republic of China , the other being Macau. A city-state situated on China's south coast and enclosed by the Pearl River Delta and South China Sea, it is renowned for its expansive skyline and deep natural harbour...

 and Macau

Macau

Macau , also spelled Macao , is, along with Hong Kong, one of the two special administrative regions of the People's Republic of China...

 are now Special Administrative Regions of China

People's Republic of China

China , officially the People's Republic of China , is the most populous country in the world, with over 1.3 billion citizens. Located in East Asia, the country covers approximately 9.6 million square kilometres...

, each maintains its own long-established postal system, which does not utilize postal codes for domestic mail, and no postal codes are assigned to Hong Kong and Macau.