Mandarin (; ) is a group of related varieties of Chinese spoken across most of northern and southwestern China. Because most Mandarin dialects are found in the north, the group is also referred to as the “northern dialect(s)”. When the Mandarin group is taken as one language, as is often done in academic literature, it has more native speakers (nearly a billion) than any other language. A northeastern-dialect speaker and a southwestern-dialect speaker may have difficulty communicating except through the standard language. Nonetheless, there is much less variation across the huge Mandarin area than between the non-Mandarin varieties of southeast China. This is attributed to the greater ease of travel and communication in the North China Plain compared to the more mountainous south, combined with the relatively recent spread of Mandarin to frontier areas. For most of Chinese history, the capital has been within the Mandarin area, making these dialects very influential. Some form of Mandarin has served as a national lingua franca since the 14th century. In the early 20th century, a standard form based on the Beijing dialect, with elements from other Mandarin dialects, was adopted as the national language. Standard Chinese, which is also referred to as “Mandarin”, is the official language of the People’s Republic of China and the Republic of China and one of the four official languages of Singapore. It is also one of the most frequently used varieties of Chinese among Chinese diaspora communities internationally.