Traditional Vietnamese music varies between the country's northern and southern regions. Northern classical music is Vietnam's oldest musical form and is traditionally more formal. The origins of Vietnamese classical music can be traced to the Mongol invasions in the 13th century when the Vietnamese captured a Chinese opera troupe. Throughout its history, Vietnam has been the most heavily impacted by the Chinese musical tradition along with those of Japan, Korea and Mongolia. ''Nhã nhạc'' is the most popular form of imperial court music, ''Chèo'' is a form of generally satirical musical theatre, while ''Xẩm'' or ''hát xẩm'' (xẩm singing) is a type of Vietnamese folk music. ''Quan họ'' (alternate singing) is popular in the former Hà Bắc Province (which is now divided into Bắc Ninh and Bắc Giang Provinces) and across Vietnam. Another form of music called ''Hát chầu văn'' or ''hát văn'' is used to invoke spirits during ceremonies. ''Nhạc dân tộc cải biên'' is a modern form of Vietnamese folk music which arose in the 1950s, while ''ca trù'' (also known as ''hát ả đào'') is a popular folk music. ''Hò'' can be thought of as the southern style of ''Quan họ''. There is a range of traditional instruments, including the ''đàn bầu'' (a monochord zither), the ''đàn gáo'' (a two-stringed fiddle with coconut body), and the ''đàn nguyệt'' (a two-stringed fretted moon lute). In recent times, there have been some efforts at mixing Vietnamese traditional music—especially folk music—with modern music to revive and promote national music in the modern context and educate the younger generations about Vietnam's traditional musical instruments and singing styles. Bolero music has gained popularity in the country since the 1930s, albeit with a different style—a combination of traditional Vietnamese music with Western elements. In the 21st century, the modern Vietnamese pop music industry known as V-pop incorporates elements of many popular genres worldwide, such as electronic, dance and R&B.