Exact use of furigana?

Question

こんにちは！

I am currently learning the Japanese language (along with the Russian language) outside of schooling, and decided that besides reading raw text for practice I could use manga as well. However, I was quite confused when I began reading my favorite series 'One Piece'.
I noticed that while I could not read most of the kanji (along with most katakana), pieces were written in hiragana to the side (which is apparently called furigana). However, furigana only appears to be besides kanji and not most katakana (despite it "consisting of smaller kana"). Can someone explain the exact usage of furigana (why it is sometimes used in rather mature series too), and what the section without furigana says.

ありがとう!

Answer 1

Hiragana and katakana are both phonetic (sound-based) writing systems. Furigana is used to show the phonetic reading of a kanji (or sometimes even Latin lettering).

It is not used with katakana because katakana is already phonetic, so its pronunciation is unambiguous. For example, my name (Eric), in katakana, would be エリック. You could express this phonetically equivalently in hiragana as えりっく. The katakana in your example would be phonetically equivalent to しゅがあ. I would recommend reading about the Japanese writing system to learn about katakana and hiragana.

On the other hand, kanji are very difficult by comparison. Students learn around 2,000 kanji over the course of many, many years, while hiragana and katakana take essentially no longer than first year. So, if the kanji is a very high grade level, the person may not know it at all, but may know the word that it represents if they know the sound of it.

Kanji also often have different readings. Notably, they usually have a Chinese reading and a Japanese reading, but they can have multiple of these as well. Take 食 for example. た and しょく are among its many readings.

In fact, the Wikipedia article you already linked states this nice and concisely:

In modern Japanese, it is mostly used to gloss rare kanji, to clarify rare, nonstandard or ambiguous kanji readings, or in children's or learners' materials.

You will notice that manga published in shounen magazines will (almost?) always have furigana, while it's far less common in seinen magazine manga, simply because by the upper end of the seinen age range, people know enough kanji to read it all. (I have noticed a few in seinen books, but as far as I know they're less common, such as the kanji for もらう.)

Answer 2

Furigana is used to clarify how a word should be pronounced. It's common in books for children, since children probably aren't familiar enough with kanji to be able to read the text. With the furigana at the side, they can figure out what the word means based on the pronunciation, while at the same time they can learn the kanji and will be able to recognize the word in the future.

Furigana can also provide rare readings that the reader probably wouldn't be able to guess without it. For example, 赤茄子 can be read as トマ��. Another example is the common word 私. This word means "I" and is usually pronounced わたし, although it can also be pronounced わたくし to make it sound more formal. If the author writes 私 but intends the わたくし reading, there's no way to convey that without furigana (except maybe context, but that's not always reliable). This reason is why you'll see it sometimes in content for adults.

It's also used a lot with names, since names often have completely irregular and unpredictable pronunciations.