Traditional tactile symbols are typically constructed by gluing a physical object to a rigid backing. The physical object may be directly connected to the symbols referent. For example, many times, a plastic spoon glued to a piece of cardboard is used to represent the concept “eat”. You’ll often see different symbol sets utilizing this “spoon = eat” metaphor. It’s much more difficult to create effective symbols for concepts that are less tangible, larger in size, or structurally complex.
Less tangible concepts tend to be modeled very differently by different tactile symbol set designers. What tactile object should be used to represent “yesterday” or “after”?
Large concepts tend to be modeled by selecting a small component of the concept even if that small component is unrepresentative of the overall concept. Should a tree be modeled as a leaf or a piece of bark?
Structurally complex concepts, similarly, are modeled by a single aspect of the concept and different designers select different components producing unavoidable ambiguity. Gym class could be modeled by choosing a accessory used during class (a ball or rope) or a piece of the environment (a piece wood to represent the court).
Traditional tactile symbol sets rarely represent more than a hundred concepts. Anytime a symbol set is constructed for a limited number of concepts, there is little motivation to base the design choices on a set of rules. As a result, there’s rarely any conceptual carryover from one symbol to another. Unrelated representations must, by definition be learned through rote memorization. This is evidenced by the fact that therapists will tell you that the object mounted to the backing isn’t as important as simply being consistent.
Probably the most telling aspect of traditional tactile symbols (just like popular graphic image sets) is the fact that they always include a text label. It’s a recognition that it’s unlikely that anyone would, a priori, connect the symbol to the concept. Only by first reading the label can someone “back into” the referent.