Rachel Whiteread's sculptures deal with the tensions between absence and presence, and the fallibility of memory. Her best known work involves making casts of vacant spaces with permanent materials such as concrete and resin, creating works that embody emptiness whilst detailing the now absent surfaces that once enclosed it. Existing as both highly visible public presentations and small, understated works, Whiteread's sculptures possess a stillness and an ability to freeze time has led to them being described as ghosts, monuments, or tombs. Untitled (Concave and Convex Beds), (1992) is part of a series of works Whiteread made that cast the forms of mattresses. Simple geometric units that flex and subtly bend, they communicate both aesthetic beauty and traces of unspecified bodies.