The most popular and widely-recognized Vietnamese national costume is the Áo Dài. Áo Dài was once worn by both genders but today it is worn mainly by females, except for certain important traditional culture-related occasions where some men do wear it. Áo Dài consists of a long gown with a slit on both sides, worn over cotton or silk trousers. Adoption and enforcement of Aó Dài took place in the mid 18th century by the rulers of Huế. They decided that their garments had to be distinctive to set themselves apart from the people of Tonkin where áo giao lĩnh and nhu quần were worn. White Áo dài is the required uniform for girls in many high schools across Vietnam. Some female office workers (e.g. receptionists, secretaries, tour guides) are also required to wear Áo Dài.