Windows1251

Windows-1251, also known as CP1251 or Windows Cyrillic, is a single-byte character encoding developed by Microsoft for representing Cyrillic script on Windows systems. It is part of the Windows code pages and consists of 256 code points. The first 128 positions correspond to ASCII, while the upper 128 are used for Cyrillic letters and a small set of punctuation and control characters.

The encoding is designed to cover languages using the Cyrillic alphabet, most notably Russian and Bulgarian, with support also for Ukrainian, Serbian, and other languages. Its layout preserves compatibility with legacy Windows software and documents created in the 1990s and 2000s; many older files and email archives use cp1251.

As a code-page based, single-byte scheme, Windows-1251 is distinct from Unicode and from other Cyrillic encodings such as ISO-8859-5 or KOI8-R. This can lead to mojibake if text encoded in cp1251 is interpreted with a different encoding. Web pages and software that declare the charset as windows-1251 will display Cyrillic text correctly on systems configured to use that encoding.

Today, the use of Windows-1251 has declined in favor of Unicode, particularly UTF-8, which provides broader language coverage and interoperability. Nevertheless, cp1251 remains common in legacy documents, emails, and certain Windows-based workflows, and conversion to UTF-8 is a common practice for modernization.

See also: Code page 1251, Cyrillic script, Windows-1251 mapping overview.