The varieties of English spoken across Great Britain form a dialect continuum, and there is no universally agreed definition of which varieties are Northern. The most restrictive definition of the linguistic North includes only those dialects spoken north of the River Tees. Other linguists, such as John C. Wells, describe these as the dialects of the "Far North" and treat them as a subset of all Northern English dialects. Conversely, Wells uses a very broad definition of the linguistic North, comprising all dialects that have not undergone the TRAP–BATH and FOOT–STRUT splits. Using this definition, the isogloss between North and South runs from the River Severn to the Wash – this definition covers not just the entire North of England (which Wells divides into "Far North" and "Middle North") but also most of the Midlands, including the distinctive Brummie (Birmingham) and Black Country dialects.