In addition to figure-ground organization, languages often allow more complex partitioning of spatial scenes. This involves segregating the ground into two reference objects in order to better locate the figure. These are termed the reference object and the secondary reference object. While the reference object (also known as the primary reference object) is usually explicitly encoded by a lexical item, the secondary reference object need not be, but can instead merely be implied, as in the following: Big Ben is north of the River Thames. While the River Thames is the primary reference object, the secondary reference object, the Earth, is implied by the spatial expression north of. In other words, it is only with respect to the concept the earth that we can process the information that one entity can be 'north of' another. The purpose of invoking a secondary reference object is to provide the primary reference object with axial properties in order to better locate the figure. This is often necessary when the primary reference object does not possess its own intrinsic asymmetry which can be employed as a reference frame.