The standard language has seven monophthong vowel phonemes. They are aligned into three vowel harmony groups by a parameter called ATR (advanced tongue root); the groups are −ATR, +ATR, and neutral. This alignment seems to have superseded an alignment according to oral backness. However, some scholars still describe Mongolian as being characterized by a distinction between front vowels and back vowels, and the front vowel spellings 'ö' and 'ü' are still often used in the West to indicate two vowels which were historically front. The Mongolian vowel system also has rounding harmony.