Diacritics

In writing, a diacritic is a mark added to a letter that means it's pronounced differently from normal, like the ticks in the e's of résumé.

In Japanese, there are two diacritics, the dakuten 濁点 and the handakuten 半濁点. They're added to the kana 仮名, the hiragana and katakana, to turn "unvoiced sounds," seion 清音, into "voiced sounds," dakuon 濁音, or "semi-voiced sounds," handakuon 半濁音.

For example: unvoiced, ka-sa-ha-ta かさはた, voiced, ga-za-ba-da がざばだ, and semi-voiced, pa ぱ.

無声音, 有声音

It's worth noting that the terms seion, dakuon and handakuon are tied to dakuten and handakuten diacritics.

The actual Japanese words for "voiced sounds" and "unvoiced sounds" would be yuuseion 有声音 and museion 無声音 respectively.

One of the key differences between the two is that dakuon and handakuon always refer to syllables, because the diacritics are always added to kana, which represent whole syllables.

The terms museion would refer to the consonants k, t, s, h, p, while yuuseion would include all the six vowels, a, i, u, e, o, n, plus the consonants g, d, z, j, b, m, n, r.^{(阿久, 1993:49)}

There are two things worth noting about this.

The dakuten isn't used in all yuuseion. For example, na な is yuuseion but has no dakuten. Since it has no dakuten, it isn't dakuon. However, every syllable with a dakuten contains a yuuseion consonant.

The handakuten is used in modern Japanese only to create pa-pi-pu-pe-po, which are museion.