The mapping between the representation and underlying referent can be:
similar (e.g., a picture of a file to represent the object file),
analogical (e.g., a picture of a pair of scissors to represent ‘cut’)
arbitrary (e.g., the use of an X to represent ‘delete’)
Most effective icons are similar ones
Many operations are actions making it more difficult to represent them
use a combination of objects and symbols that capture the salient part of an action