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+# Standardized codes
+
+Many common concepts, such as spoken languages, countries, currency, and so on,
+have common codes (usually formalized by the [International Organization for
+Standardization][iso]) that are used in data communication and processing.
+These codes address the issue that there are often different ways to express
+the same concept in written language (for example, "United States" and "USA",
+or "Español" and "Spanish").
+
+## Guidance
+
+For concepts where a standardized code exists and is in common use, fields
+representing these concepts **should** use the standardized code for both input
+and output.
+
+```typescript
+// A message representing a book.
+interface Book {
+  // Other fields...
+
+  // The IETF BCP-47 language code representing the language in which
+  // the book was originally written.
+  // https://en.wikipedia.org/wiki/IETF_language_tag
+  languageCode: string;
+}
+```
+
+- Fields representing standardized concepts **must** use the appropriate data
+  type for the standard code (usually `string`).
+  - Fields representing standardized concepts **must** indicate which standard
+    they follow, preferably with a link (either to the standard itself, the
+    Wikipedia description, or something similar).
+- The field name **should** end in `_code` or `_type` unless the concept has an
+  obviously clearer suffix.
+- When accepting values provided by users, validation **should** be
+  case-insensitive unless this would introduce ambiguity (for example, accept
+  both `en-gb` and `en-GB`). When providing values to users, APIs **should**
+  use the canonical case (in the example above, `en-GB`).
+- Standardized code fields **may** have a default; if they do, the sentinel
+  value **must** be the omission of the field.
+
+**Note:** The string itself _is_ the immutable, canonical code, used on the
+wire format. Services **should not** create a separate enum with a different
+wire format, and **should not** use company-specific strings, because doing so
+makes it difficult to use multiple APIs together.
+
+### Content types
+
+Fields representing a content or media type **must** use [IANA media types][].
+For legacy reasons, the field **should** be called `mime_type`.
+
+### Countries and regions
+
+Fields representing individual countries or nations **must** use the [Unicode
+CLDR region codes][cldr] ([list][]), such as `US` or `CH`, and the field
+**must** be called `region_code`.
+
+**Important:** We use `region_code` and not `country_code` to include regions
+distinct from any country, and avoid political disputes over whether or not
+some regions are countries.
+
+### Currency
+
+Fields representing currency **must** use [ISO-4217 currency codes][iso-4217],
+such as `USD` or `CHF`, and the field **must** be called `currency_code`.
+
+**Note:** For representing an amount of money in a particular currency, rather
+than the currency code itself, use [`google.protobuf.Money`][money].
+
+### Language
+
+Fields representing spoken languages **must** use [IETF BCP-47 language
+codes][bcp-47] ([list][]), such as `en-US` or `de-CH`, and the field **must**
+be called `language_code`.
+
+### Time zones
+
+Fields representing a time zone **should** use the [IANA TZ][] codes, and the
+field **must** be called `time_zone`.
+
+Fields also **may** represent a UTC offset rather than a time zone (note that
+these are subtly different). In this case, the field **must** use the [ISO-8601
+format][] to represent this, and the field **must** be named `utc_offset`.
+
+## Changelog
+
+- **2020-05-12**: Replaced `country_code` guidance with `region_code`,
+  correcting an original error.
+
+<!-- prettier-ignore-start -->
+[bcp-47]: https://en.wikipedia.org/wiki/IETF_language_tag
+[cldr]: http://cldr.unicode.org/
+[iana media types]: https://www.iana.org/assignments/media-types/media-types.xhtml
+[iana tz]: http://www.iana.org/time-zones
+[iso]: https://www.iso.org/
+[iso-4217]: https://en.wikipedia.org/wiki/ISO_4217
+[iso-8601 format]: https://en.wikipedia.org/wiki/ISO_8601#Time_offsets_from_UTC
+[list]: https://www.iana.org/assignments/language-subtag-registry/language-subtag-registry
+[money]: https://github.com/googleapis/api-common-protos/blob/master/google/type/money.proto
+<!-- prettier-ignore-end -->
diff --git a/aip/general/0143/aip.yaml b/aip/general/0143/aip.yaml
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+---
+id: 143
+state: approved
+created: 2019-07-24
+placement:
+  category: fields
+  order: 40