 To explain the division of labor between text and diagram in ancient geometry, Manders distinguishes between the exact and co-exact properties of geometric diagrams in Manders  2008 [1995].  Underlying the distinction is a notion of variation. The co-exact conditions realized by a diagram ‘are those conditions which are unaffected by some range of every continuous variation of a specified diagram.’ Exact conditions, in contrast, are affected once the diagram is subject to the smallest variation. Roughly, a diagram’s co-exact properties comprise the ways its parts define a finite set of planar regions, and the containment relations between these regions. A prominent exact relation is the equality of two magnitudes within a diagram. For example, only the slightest change to the position of CF in the diagram for proposition 16 is required to make the angles BAE and ECF unequal.