The ~200 km thick layer of the lower mantle directly above the boundary is referred to as the D’’ ("D double-prime" or "D prime prime") and is sometimes included in discussions regarding the core–mantle boundary zone. The D’’ name originates from the mathematician Keith Bullen's designations for the Earth's layers. His system was to label each layer alphabetically, A through G, with the crust as 'A' and the inner core as 'G'. In his 1942 publication of his model, the entire lower mantle was the D layer. In 1950, Bullen found his "D" layer to actually be two different layers. The upper part of the D layer, about 1800 km thick, was renamed D’ (D prime) and the lower part (the bottom 200 km) was named D’’.