UML: an evaluation of the visual syntax of the language

Examination of the UML indicates weaknesses in its graphic syntax which undermine its structure as a visual language. Although the UML Notation claims to provide a "canonical notation", there are insufficient rules governing the graphic constructs used to produce the essential \´signifiers\´ of this visual language and to define their permissible combinations. The nature and composition of the graphical elements actually shown is a fundamental consideration, separate from the underlying constructs that they may signify. A much earlier formulation for notational systems, that provided by Nelson Goodman, clarifies the issues involved and makes it possible to set basic tests for a notational scheme, such as the UML, which require syntactic disjointness and differentiability. Application of these tests (plus others) to graphical primitives, simple characters and diagrams shows a variety of failures that lead to a fundamental questioning of the graphical syntax which forms part of the UML structure as a language.