Complex sentence: The spelling differences between the four kana were retained well up to the mid-twentieth century, long after the different sounds they represented had merged leaving only two distinct morae in most mainland dialects such as that of Tokyo.
Split into simpler sentences: The spelling differences between the four kana were retained well up to the mid-twentieth century, long after the different sounds they represented had merged. This left only two distinct morae in most mainland dialects, such as that of Tokyo.