Wikipedia
The , also known as , , , or Katakana-Hiragana Prolonged Sound Mark by the Unicode Consortium, is a Japanese symbol which indicates a chōon, or a long vowel of two morae in length. Its form is a horizontal or vertical line in the center of the text with the width of one kanji or kana character. It is written horizontally in horizontal text and vertically in vertical text. The chōonpu is usually used to indicate a long vowel sound in katakana writing, rarely in hiragana writing, and never in romanized Japanese. The chōonpu is a distinct mark from the dash, and in most Japanese typefaces it can easily be distinguished. In horizontal writing it is similar in appearance to, but should not be confused with, the kanji character ("one").
The symbol is sometimes used with hiragana, for example in the signs of ramen restaurants, which are normally written in hiragana. Usually, however, hiragana does not use the chōonpu but another vowel kana to express this sound. The following table shows the usual hiragana equivalents used to form a long vowel, using the ha-gyō (the ha, hi, fu, he, ho sequence) as an example.
Romaji
Hiragana
Katakana
hā
hī
fū / hū
hē (hee)
hō (hoo)
When rendering English words into katakana, the chōonpu is often used to represent a syllable-final sequence of a vowel letter + r, which in English generally represents a long vowel if the syllable is stressed and a schwa if unstressed (in rhotic dialects it may additionally be an R-coloured vowel). For example, both "ar" and "er" are usually represented by a long ā vowel, with the words "number" and "car" becoming (nanbā) and (kā).
In addition to Japanese, chōonpu are also used in Okinawan writing systems to indicate two morae. The Sakhalin dialect of Ainu also uses chōonpu in its katakana writing for long vowels.
In Unicode, the chōonpu has the value U+30FC , which corresponds to JIS X 0208 kuten code point 01-28, encoded in Shift JIS as 815F. It is normally rendered fullwidth and with a glyph appropriate to the writing direction. The halfwidth compatibility form has the value U+FF70 , which is converted to Shift JIS value B0.