[c] For this last row, it supports letters for Belarusian and Ukrainian in addition to Russian, but in a layout unrelated to code page 866 or 1125.

[11] The other variant, KOI-8 N1, is a subset of KOI-8 N2 which omits the non-Russian Cyrillic letters and mixed single/double lined box-drawing characters, leaving them empty for further internationalization (compare with code page 850).

[20] Ukrainian standard RST 2018-91 is designated by IBM as Code page 1125 (CCSID 1125),[25] abbreviated CP1125, and also known as CP866U, CP866NAV or RUSCII.

[26] It matches the original Alternative code page for all points except for F2hex through F9hex inclusive, which are replaced with Ukrainian letters.

Also, the so-called CP 866ukr code page is a modified version of CP866 with the replacement of Ўў by Іі.

It is not included in the standard Windows distributions, but some users install a home-made patch[38] that allows using this encoding to work in command-line programs (such as FAR Manager) with filenames containing the Cyrillic Іі.

It replaces three mathematic symbols with guillemets and the section sign which are commonly used in the Russian language.

(Lehner and Czech created a number of alternative character sets for other European languages as well, including one based on CWI-2 for Hungarian, a Kamenicky-based one for Czech and Slovak, a Mazovia variant for Polish and a seemingly-unique encoding for Lithuanian.

[48] FreeDOS provides additional unofficial extensions of code page 866 for various non-Slavic languages:[49] Before Microsoft's final code page for Russian MS-DOS 4.01 was registered with IBM by Franz Rau of Microsoft as CP866 in January 1990, draft versions of it developed by Yuri Starikov (Юрий Стариков) of Dialogue were still called code page 900 internally.

While the documentation was corrected to reflect the new name before the release of the product, sketches of earlier draft versions still named code page 900 and without Ukrainian and Belarusian letters, which had been added in autumn 1989, were published in the Russian press in 1990.

[50] Code page 900 slipped through into the distribution of the Russian MS-DOS 5.0 LCD.CPI codepage information file.