Conservative vowel systems, such as those of Gan dialects, have high vowels , and , which also function as medials, mid vowels and , and a low -like vowel. In other dialects, including Mandarin dialects, has merged with , leaving a single mid vowel with a wide range of allophones. Many dialects, particularly in northern and central China, have apical or retroflex vowels, which are syllabic fricatives derived from high vowels following sibilant initials. In many Wu dialects, vowels and final glides have monophthongized, producing a rich inventory of vowels in open syllables. Reduction of medials is common in Yue dialects.