>>171
Ok, finally someone asked for it, I was about to write an article on this issue. Anyway here is the pattern.
When two on-yomi kanji combine to become a 熟語(jyukugo), if the kana of the first kanji ends with つ(e.g.発~はつ 実~じつ 說~せつ 必~ひつ) or ち(e.g. 一~いち, 日~にち), the first kanji becomes 促音(sokuon) according to the rules as follows.
Rule no.1: when combine with the second kanji that begins with kana belongs to kana columns that have no 濁音(dakuon), the first kanji does NOT become 促音(sokuon).
e.g. 説明(せつめい),発明(はつめい), the kana ま of 明(めい) belongs to ま行(ma column) and has no 濁音(dakuon), therefore 説(せつ) and 発(はつ) do not become 促音(sokuon).
Rule no.2: When combine with the second kanji that begins with kana belongs to kana columns that have 濁音(dakuon),
A>> If the kana of the second kanji begins with 濁音(dakuon), the first kanji ALSO does NOT become 促音(sokuon).
e.g. 発動(はつどう), 実在(じつざい), the kana どof 動(どう) and ざof 在(ざい) are 濁音(dakuon) therefore 発(はつ) and 実(じつ) do not become 促音(sokuon).
B>> If the kana of the second kanji begins with 清音(seion) that has 濁音(dakuon), the first kanji MUST become 促音(sokuon).
e.g. 実験(じっけん), 実行(じっこう), the kana けof 験(けん) and こof 行(こう) belong to か行清音(seion of ka column) and か行(ka column) has 濁音(dakuon) therefore 実(じつ) MUST become 促音(sokuon) and pronounce じっ.
Rule no.3: When combine with the second kanji that begins with one of the は ひ ふ へ ほ(seion of ha column) five kana, follow rule no.2 B>> PLUS the first kana of the second kanji MUST become 半濁音(hadakuon).
e.g. 脱皮, 出発
Sorry for my clumsy writing but the point is to understand the pattern, not to learn the rules by rote.