The double acute accent is mainly used in Hungarian.

Erdős's name contains the relatively uncommon character "ő" ("o" with double acute accent).

Double acute accent (ő)

The double acute accent probably found its way by analogy from Latin script languages (probably German) - cf. ger.

O and U with double acute accents are supported in the ISO 8859-2 and Unicode character sets.

Note that, although Kőnig's name is properly spelled with a double acute accent, the theorem named after him is customarily spelled with an umlaut.

Due to character encoding confusion, the letters can be seen on many incorrectly coded Hungarian web pages, representing Ő/ő (letter O with double acute accent).

In several Latin script languages where umlaut (diacritic) is used, but double acute accent is not (i.e., there is no danger of mixing them up) handwriting umlaut may look like double acute accent.