Per Mollerup's book Marks of Excellence thoroughly studies trademarks, their design, functions and history. The function of trademarks is of course different than that of icons, but there are many similarities in the aspects of their design. Mollerup has also created a complete taxonomy system for trademarks.
Studying this system gives a good point of reference for the development of an icon taxonomy. According to Mollerup there are five rules a taxonomy must comply with, although as he points out, even his own system doesn´t comply with all the rules.
- It must consist of classes that are distinct. The differences between the classes must be clear so there is no room for misunderstanding to which class an item belongs to.
- The characteristics the classes are based on should be used consistently and each step in the classification should be based on a single principle of division.
- There should be no overlapping between classes. Parallel (co-ordinate) classes should be exclusive.
- Co-ordinate classes should be able to collectively cover all possible entries.
- The classes should be relevant to the purpose of the taxonomy.
Mollerup's system is described below.
1.1) Graphic marks
1.1.1) Picture marks
126.96.36.199) Figurative marks
188.8.131.52.1) Descriptive marks
184.108.40.206.2) Metaphoric marks
220.127.116.11.3) Found marks
18.104.22.168) Non-figurative marks
1.1.2) Letter marks
22.214.171.124) Name marks
126.96.36.199.1) Proper names
188.8.131.52.2) Descriptive names
184.108.40.206.3) Metaphoric names
220.127.116.11.4) Found names
18.104.22.168.5) Artificial names
22.214.171.124.1) Initial abbreviations
126.96.36.199.1.2) Non-acronym initial abbreviations
188.8.131.52.2) Non-initial abbreviations
1.2) Non-graphic marks
Icons rarely consist only of letters but sometimes letters and words may be a part of an icon. Thereby branch 1.1.1.) Picture marks is the most interesting for the purpose of icon taxonomy. If we examine the subclasses it contains, figurative marks are marks that depict an object and non-figurative marks are pictures in their own right. Non-figurative icons would be icons that appear completely abstract. For instance the stop icon by itself might be considered non-figurative. Also icons whose graphical meanings are unclear to a user, might be considered non-figurative. The power icon for instance is a logical graphical reference, but to some users it might seem completely abstract.
Descriptive marks refer directly to their object. The printer icon is a clear examle of a descriptive icon. Metaphoric marks refer to their object through a shared quality. The file and thrash can are metaphoric icons. Found marks show something recognizable that is arbitrary to its object. The relationship of the symbol and its object might often have a historical explanation that is logical. Over time the connection is lost and the symbol becomes arbitrary. The floppy disk as an icon for the save function is becoming arbitrary to younger users who have never used the storage media, and thereby it might be considered a found mark.
Mollerup, P. (1997). Marks of Excellence. London. Phaidon Press Limited.