Spoken Chinese comprises many regional variants, generally referred to as dialects. However, the mutual unintelligibility of the subvarieties is the main ground for classifying them as separate languages or dialect groups. Each dialect group consists of a large number of dialects, many of which may themselves be referred to as languages. The boundaries between one so-called language and the next are not always easy to define. Because each dialect group preserves different features of Middle Chinese (dating back to early or even pre-T'ang times), they have proven to be valuable research tools in the phonological reconstruction of Middle and even to some extent its ancestor, Old Chinese. Most Chinese speak one of the Mandarin dialects, which are largely mutually intelligible.