Postal Address

A postal address tells a carrier where to deliver something. It looks like simple structured data, a path from country down to building, but addresses are among the most irregular data that software routinely collects.

The usual model is a hierarchy: country, region, city, street, number, and unit. Real addresses break it in many ways:

• Order differs. Many countries write small to large, house before street before city; Japan and China write from large to small.
• Streets aren't always the key. Most Japanese addresses number city blocks and buildings, not streets, and many streets have no names.
• Many places lack formal addresses. Rural areas and informal settlements often rely on landmarks, and directions in Costa Rica traditionally count meters from a known building.
• Fields vary. Some countries have no states or provinces, and some have no postal codes.

Postal codes were added to speed sorting. Germany introduced numbered postal zones in 1941; the US ZIP Code, for Zone Improvement Plan, arrived in 1963, and nine-digit ZIP+4 codes in 1983; the UK rolled out alphanumeric postcodes nationwide between 1959 and 1974; and Ireland had almost none outside Dublin until it launched Eircode in 2015, giving each address its own code. A US ZIP code is a set of delivery routes, not an area with a boundary; it can cross county or state lines, and one large building can have its own. The Census Bureau builds approximate areas for them precisely because ZIP codes don't define any.

For anyone designing a form or schema:

1. Ask for the country first, then show the fields that country uses.
2. Prefer address lines to rigidly split fields, and don't require a state or postal code where none exists.
3. Accept any script, and store text in a universal encoding; see Character Encoding.
4. Offer lookup as the user types, which fixes spelling; see Autocomplete.
5. Verify against postal data when delivery matters, rather than inventing regex rules.

Addresses are also a common key for matching records about the same household or property, but the same place can be written many ways, so matching needs standardization first. Converting them to coordinates is a separate step.

Mail that can't be delivered goes to a dead letter office, a name software borrowed for undeliverable messages. Email took the word "address" too, writing its two parts as person at domain.

People without a fixed address, including those who are homeless, often struggle to open bank accounts or receive benefits, a reminder that the field encodes who a system assumes its users are.