The tones of Middle Chinese, as well as similar systems in neighbouring languages, experienced a tone split conditioned by syllabic onsets. Syllables with voiced initials tended to be pronounced with a lower pitch, and by the late Tang Dynasty, each of the tones had split into two registers conditioned by the initials, known as "upper" (/ "yīn") and "lower" (/ "yáng"). When voicing was lost in all dialects except the Wu and Old Xiang groups, this distinction became phonemic, yielding eight tonal categories, with a six-way contrast in unchecked syllables and a two-way contrast in checked syllables. Cantonese maintains these eight tonal categories and has developed an additional distinction in checked syllables. (The latter distinction has disappeared again in many varieties.)