Why are thereand?

Long long time ago, until Kamakura era,andwere different sounds. was [Ʒi] andwas [di]. So wereand. was [zu] and was [du]. But during Muromachi era, these sounds changed.andbecame the same sound and so didand. During Genroku era even a book was published to tell people which of the two letters they should use for confusing words. As the correct use of these letters has been a problem, they are called "Yotsugana (the four letters)". Nowadays, the rule is simple. In general you only useor. However, there are two cases you have to use or. 1) If a word is originally made from two words and the latter one has or. 　　 e.g. +=　　　　+= 2) When a word srats withand [dƷi] sound follows. When a word starts withand [zu] sound follows. 　　 e.g.