The official and majority language is Lao, a language of the Tai-Kadai language family. However, only slightly more than half of the population speaks Lao natively. The remainder, particularly in rural areas, speak ethnic minority languages. The Lao alphabet, which evolved sometime between the 13th and 14th centuries, was derived from the ancient Khmer script and is very similar to Thai script. Languages like Khmu (Austroasiatic) and Hmong (Hmong-Mien) are spoken by minorities, particularly in the midland and highland areas. A number of Laotian sign languages are used in areas with high rates of congenital deafness.