Can you guess what the purpose of the book entitled 蜆縮涼鼓集, Ken-shuku-ryō-ko-shuū, or "Clam shrink cool drum collection", was? It's right there in the title. Right there.

Answer: it was a late 17th-century book listing words that used the "ji" (じ) in "shijimi" (しじみ, 蜆, a type of bivalve) versus the "ji" (ぢ) in "chijimi" (ちぢみ, 縮, shrink[age], plus various related meanings), and words that used the "zu" (ず) in "suzumi" (すずみ, 涼み, cool[ness]) versus the "zu" (づ) in "tsuzumi" (つづみ, 鼓, drum). Readers used it to check spellings of words using these dangerous sounds.

[So why not call it the Shijimi-chijimi-suzumi-tsuzumi-shuu? Well, for one thing, no-one would be able to pronounce that title properly. Ha! Seriously, for the same reason that the Tokyo (東京)-Yokohama (横浜)-Chiba (千葉) area becomes "Keihinyō" (京浜葉) in abbreviation: Kanji-based abbreviations traditionally got the Sino-Japanese reading, I suppose analogous to English speakers saying "L.A." instead of "Lo'-A'".]

This was right about the time that the last traces of the difference between each pair were disappearing from the spoken language, but still centuries before any organized orthographic reform would be undertaken. Today's Japanese speakers (when writing) still have to know when to use じ and when to use ぢ, etc., but since most spellings have been normalized, you can get away with a few simple rules and a handful of exceptions instead of memorizing a whole book.