Interpretation of graphics is "loosely" analogous to the process of reading text, while generation of graphics is the counterpart of writing text. However, these analogies are imperfect, as text and graphics are based on very different symbol systems. For example, whereas text is structured according to formal organisational rules that apply irrespective of the content, this is not the case for graphics. With text structure, the units of information (words) are expected to be organised according to broad conventions (such as being sequenced in orderly rows starting from top left and progressing down the page). However graphics are not subject to a similarly stringent set of structural conventions. Instead, it is the content itself that largely determines the nature of the graphic entities and the way they are arranged. For example, the form and spatial arrangement of the items that comprise the actual subject matter being represented in the graphic are used as the basis for the graphic entities and structure that are displayed in the graphic. This is not the case with written text where the words and their arrangement bear no resemblance to the represented subject matter.