Another example is stage blocking, where "stage left" "stage right" "stage up" and "stage down" are, by convention, defined from the actor's point of view, but up and down stage do not follow gravitational conventions of up and down. An example of a non-egocentric view is page layout, where the relative terms "upper half" "left margin," etc. are defined in terms of the observer but employed in reverse for a type compositor, returning to an egocentric view. In medicine and science, where precise definitions are crucial, relative directions (left and right) are the sides of the organism, not those of the observer.