Peirce [...] singled out two distinct subclasses of icons - "images" and "diagrams". In images the signans represents the "simple qualities" of the signatum, whereas for diagrams the likeness between the signans and signatum exists "only in respect to the relations of their parts". Peirce defined a diagram as "a 'representamen' which is predominantly an icon of relation and is aided to be so by conventions".
- Jakobson (2004), Pag. 18