The horn is a diacritic

Diacritic

A diacritic is a glyph added to a letter, or basic glyph. The term derives from the Greek διακριτικός . Diacritic is both an adjective and a noun, whereas diacritical is only an adjective. Some diacritical marks, such as the acute and grave are often called accents...

 mark attached to the top right corner of the letters o and u in the Vietnamese alphabet

Vietnamese alphabet

The Vietnamese alphabet, called Chữ Quốc Ngữ , usually shortened to Quốc Ngữ , is the modern writing system for the Vietnamese language...

 to give ơ and ư, unrounded variants of the vowel

Vowel

In phonetics, a vowel is a sound in spoken language, such as English ah! or oh! , pronounced with an open vocal tract so that there is no build-up of air pressure at any point above the glottis. This contrasts with consonants, such as English sh! , where there is a constriction or closure at some...

 represented by the basic letter. In Vietnamese, it is rarely considered a separate diacritic; rather, the characters ơ and ư are considered separate from o and u.

Encodings

In Unicode

Unicode

Unicode is a computing industry standard for the consistent encoding, representation and handling of text expressed in most of the world's writing systems...

, horn is encoded as a combining mark, and precomposed letters.
The Unicode and HTML codes

Character encodings in HTML

HTML has been in use since 1991, but HTML 4.0 was the first standardized version where international characters were given reasonably complete treatment...

 for letters with a horn are:
and

See also

• Ơ
  O
  O is the fifteenth letter and a vowel in the basic modern Latin alphabet.The letter was derived from the Semitic `Ayin , which represented a consonant, probably , the sound represented by the Arabic letter ع called `Ayn. This Semitic letter in its original form seems to have been inspired by a...
  
  
• Ư
  U
  U is the twenty-first letter and a vowel in the basic modern Latin alphabet.-History:The letter U ultimately comes from the Semitic letter Waw by way of the letter Y. See the letter Y for details....
  
  
• Hook (diacritic)
  Hook (diacritic)
  In typesetting, the hook is a diacritic mark placed on top of vowels in the Vietnamese alphabet. In shape it looks like a tiny question mark without the dot underneath. For example, a capital A with a hook is "Ả", and a lower case "u" with a hook is "ủ". It functions as a tone marker...
  
  
• Acute accent
  Acute accent
  The acute accent is a diacritic used in many modern written languages with alphabets based on the Latin, Cyrillic, and Greek scripts.-Apex:An early precursor of the acute accent was the apex, used in Latin inscriptions to mark long vowels.-Greek:...
  
  
• Apostrophe
  Apostrophe
  The apostrophe is a punctuation mark, and sometimes a diacritic mark, in languages that use the Latin alphabet or certain other alphabets...