Years ago I did an internship at a foreign company in Japan whose local branch had the somewhat enigmatic extension "K.K." Throughout the internship I kept wondering what these two letters were supposed to mean, but found it just too embarrassing to ask.

I'm mentioning this episode because it relates — with some detours — to an interesting phenomenon in Japanese word formation. When two words melt into one, they frequently seal their morphological relationship with a "blurring" of the second part. Take hana-bi (花火, firework), for instance, which is a compound of hana (花, flower) and hi (火, fire). This process, which turns hi into bi, ku into gu, chi into ji, etc., is commonly known as rendaku (連濁), a connection (連) that blurs (濁).

Rendaku is particularly frequent in native Japanese words, as in our hana-bi example. Sino-Japanese vocabulary is known to resist rendaku, though on closer inspection this turns out to be not quite in line with the facts. True, it's kakueki-teisha (各駅停車, local train) and shōmei-shashin (証明写真, ID photo) rather than kakueki-deisha and shōmei-jashin (no matter how blurred the photo is). But on the other hand, there are quite a number of Sino-Japanese words that do blur: renshū-jiai (練習試合, practice game), doryoku-busoku (努力不足, lack of effort) and wa-gashi (和菓子, Japanese-style confectionery), to name but a few. And rendaku is very common in Sino-Japanese two-character words such as renpai (連敗, succession of defeats), senpai (先輩, one's senior) and kanpai (乾杯, cheers).