In 1938, a letter shaped similarly to heng was approved as the official IPA letter for this sound. It replaced . However, the International Phonetic Association also suggested that a compromise between the two letters could be used if the author wanted. This compromise letter was included in the 1949 "Principles of the International Phonetic Association" and the IPA charts of that year. Although the International Phonetic Association said to use this letter, some authors still used from the 1960s to the 1980s. Later, this new letter was replaced by at the 1989 Kiel Convention.