A datum is a consistent line carried through the field of vision. In the design of a home, it might be window sills aligned throughout a room, or a picture rail that runs around the upper walls of an apartment. Check out these examples, both contemporary and traditional.
A datum can create order, continuity, and cohesiveness, all of which can be helpful characteristics in any space, but particularly effective in small spaces.
Are there any clear datum lines in your home? What are they? Was a datum already established when you moved in or is it something you created yourself?