Language Atlas of China
Of Yuan's four Mandarin subgroups, the Northwestern dialects are the most diverse, particularly in the province of Shanxi. The linguist Li Rong proposed that the northwestern dialects of Shanxi and neighbouring areas that retain a final glottal stop in the Middle Chinese entering tone (plosive-final) category should constitute a separate top-level group called Jin. He used this classification in the "Language Atlas of China" (1987). Many other linguists continue to include these dialects in the Mandarin group, pointing out that the Lower Yangtze dialects also retain the glottal stop.