Furigana may be added by character, in which case the part of a word which corresponds to a kanji is centered over that kanji; or by word or phrase, in which case the entire furigana word is centered over several characters, even if the kanji do not represent equal shares of the kana needed to write them. The latter method is more common, especially since some words in Japanese have unique pronunciations that are not related to any of the characters the word is written with.
When it is necessary to distinguish between native Japanese and Chinese-derived pronunciations, for example in kanji dictionaries, the Japanese pronunciations are written in hiragana, and the Chinese ones are written in katakana. However, this distinction is really only important in dictionaries and other reference works. In ordinary prose, the script chosen will usually be hiragana. The one general exception to this is modern Chinese place names, personal names, and (occasionally) food names — these will often be written with kanji, and katakana used for the furigana.
The distinction between regular kana and the smaller character forms, which are used in regular orthography to mark such things as gemination and palatalization, is often not made in furigana: for example, the usual hiragana spelling of the word 却下 (kyakka) is きゃっか, but in furigana it might be written きやつか. This was especially common in old-fashioned movable type printing when smaller fonts were not available. Nowadays, with computer-based printing systems, this occurs less frequently.
Furigana are most commonly used in works for children, who may not have sufficiently advanced reading skills to recognise the kanji, but can understand the word when written phonetically in hiragana. Because children learn hiragana before katakana, in books for very young children, there are hiragana furigana next to the katakana characters. It is common to use furigana on all kanji characters in works for young children. This is called in Japanese.
There are also a lot of manga for adolescents, which use furigana. There are also books with a phonetic guide (mainly in Hiragana but sometimes in Romaji) for Japanese learners, which may be bilingual or Japanese only, they are popular with foreigners, wishing to master Japanese faster and enjoy reading Japanese short-stories, novels or articles.
Some web sites and tools exist, which provide a phonetic guide in Japanese web pages (in Hiragana, Romaji or Kiriji), they are popular both with Japanese children and foreign Japanese learners.
In works aimed at adult Japanese speakers, furigana may be used on a word written in uncommon kanji; in the mass media, they are generally used on words containing non-Jōyō kanji.
Furigana commonly appear alongside kanji names and their romanizations on signs for railway stations, even if the pronunciation of the kanji is commonly known. Furigana also appear often on maps to show the pronunciation of unusual place names.
Japanese names are usually written in kanji. Because there are many possible readings for kanji names, including special name-only readings called nanori, furigana are often used to give the readings of names. On Japanese official forms, where the name is to be written, there is always an adjacent column for the name to be written in furigana. Usually katakana is preferred.
Furigana may also be used for foreign names written in kanji. Chinese and Korean names are the most common examples: Chinese names are usually pronounced with Japanese readings and the pronunciation written in hiragana, while Korean names are usually pronounced with Korean readings and the pronunciation written in katakana. Furigana may also be necessary in the rare case where names are transliterated into kanji from other languages (e.g., soccer star Ruy Ramos and activist Arudou Debito).
Kanji and kanji compounds are often presented with furigana in Japanese language textbooks for non-native speakers.
Furigana are also often used in foreign language textbooks for Japanese learners to indicate pronunciation. The words are written in the original foreign script, such as hangul for Korean, and furigana is used to indicate the pronunciation.
Some writers use furigana to represent slang pronunciations, particularly those which would become hard to understand without the kanji to provide their meaning.
Another use is to write the kanji for something which had been previously referenced, but write furigana for "" (それ) or "" (あれ), meaning "that", indicating that the characters simply refer to it with a pronoun, but clarifying for the reader what thing was meant.
In karaoke it is extremely common for furigana to be placed on the song lyrics. The song lyrics are often written in kanji pronounced quite differently from the furigana. The furigana version is used for pronunciation.
Also, because the kanji represent meaning while the furigana represent sound, one can combine the two to create puns or indicate meanings of foreign words. One might write the kanji for "blue", but use katakana to write the pronunciation of the English word "blue"; this may be done, for example, in Japanese subtitles on foreign films, where it can help associate the written Japanese with the sounds actually being spoken by the actors, or it may be used in a translation of a work of fiction to enable the translator to preserve the original sound of a proper name (such as "Firebolt" in the Harry Potter series) in furigana, while simultaneously indicating its meaning with kanji. A similar practice is used in native fiction to produce double meanings: for example, the word for "Earth" might be written with furigana for "homeland" (ふるさと) as the reading in a work of science fiction.
In the written style known as kanbun, which is the Japanese approximation of Classical Chinese, small marks called kunten are sometimes added as reading aids. Unlike furigana, which indicate pronunciation, kunten indicate Japanese grammatical structures absent from the kanbun, as well as showing how words should be reordered to fit Japanese sentence structure.
Furigana are sometimes also used to indicate meaning, rather than pronunciation. Over the foreign text smaller sized Japanese words, in kana or kanji, corresponding to the meaning of the foreign words, effectively translate it in place. While rare now, some late 19th–early 20th century authors used kanji as furigana for loanwords written in katakana. This usage is called in Japanese, since furigana implies the use of kana.