In 1998 the Tate acquired 26 sheets from a spiral-bound sketchbook originally given to friends by the painter Francis Bacon (1909-1992) in 1961.
Thinned-down oil paint was the main medium for these sketches. Hasty brushwork. Also sometimes ballpoint or pencil.
The curators remark that most of the sketches do not correspond to known paintings completed by Bacon. Yet the archetypes on display here are all familiar ones. To me it makes sense that Bacon was not so much conjuring specific poses as studying force fields.