The horn (Vietnamese: dấu móc or dấu râu) is a diacritic mark attached to the top right corner of the letters o and u in the Vietnamese alphabet to give ơ and ư, unrounded variants of the vowel represented by the basic letter. In Vietnamese, they are rarely considered as diacritics; but rather, the characters ơ and ư are considered as different letters from o and u.

In Unicode, horn is encoded as a combining mark, and precomposed letters.

• U+031B  ̛  COMBINING HORN

Precombined characters in Unicode and HTML codes:

• U+01A0 Ơ LATIN CAPITAL LETTER O WITH HORN
• U+01A1 ơ LATIN SMALL LETTER O WITH HORN

and

• U+01AF Ư LATIN CAPITAL LETTER U WITH HORN
• U+01B0 ư LATIN SMALL LETTER U WITH HORN

• Hook (diacritic)
• Acute accent
• Apostrophe