Local varieties from different areas of China are often mutually unintelligible, differing at least as much as different Romance languages and perhaps even as much as Indo-European languages as a whole. These varieties form the Sinitic branch of the Sino-Tibetan language family (with Bai sometimes being included in this grouping). Because speakers share a standard written form, and have a common cultural heritage with long periods of political unity, the varieties are popularly perceived among native speakers as variants of a single Chinese language, and this is also the official position. Conventional English-language usage in Chinese linguistics is to use "dialect" for the speech of a particular place (regardless of status) while regional groupings like Mandarin and Wu are called "dialect groups". ISO 639-3 follows the "Ethnologue" in assigning language codes to eight of the top-level groups listed above (all but Min and Pinghua) and five subgroups of Min. Other linguists choose to refer to the major groupings as languages. Sinologist David Moser stated that the Chinese authorities refer to them as "dialects" as a way to reinforce China as being a single nation.