Complex sentence: Today, with a few exceptions, there is only one hiragana for each of the fifty consonant--vowel sequences (moras) in Japanese, but before 1900, there were generally several more-or-less interchangeable hiragana for each.
Split into simpler sentences: Today, with a few exceptions, there is only one hiragana for each of the fifty consonant--vowel sequences (moras) in Japanese. However, traditionally there were generally several more-or-less interchangeable hiragana for each.