dakuten 濁点, ゛

The dakuten 濁点, sometimes called tenten てんてん, chonchon ちょんちょん, or dakuonpu 濁音符, are diacritics, accents used on kana to represent a "voice sound," a dakuon 濁音. They look like two small diagonal marks ゛ on the top right of the kana. For example: ga が is ka か with dakuten.

The dakuten are applied to the consonants to turn them into voice consonants. It's used to turn K-S-T-H into G-Z-D-B. The diacritic that turns H into P, and looks like a circle ゜, is called handakuten 半濁点, literally "half" dakuten.

Source: japanesewithanime.com (CC BY-SA 4.0)

Different Words

Two words with the same kana are different words if one has dakuten and the other does not. For example:

• shinsha 新車
  New car.
• shinja 信者
  Believer.
• jinsha 仁者
  Person of virtue.
• jinja 神社
  Shinto shrine.

Rendaku

Sometimes the first syllable of a word becomes voiced when it's suffixed to another word. This phenomenon is called rendaku 連濁. For example:

• hito 人
  Person.
• koibito 恋人
  Lover. Person who's loved.

Strange Tenten Usage

Besides its standard, normal use in turning consonants into voiced consonants, the dakuten can also e used make virtually any kana voiced.

Notably, vu ヴ, a voiced u, isn't found in native Japanese words, only in loan-words, and, consequentially, it can only be typed in a computer in katakana, not in hiragana. (although you could type the dakuten separately: う゛)

Artistically, in manga, light novels, etc. the dakuten may be added to random kana when a character is screaming in order to denote added stress to the pronunciation. This is often done together with small kana.

Example: aaaahhhh ああああ゛あ゛ぁ゛ぁ゛