Why the diagram leaves things out

An original exposition about an invented information-design exercise.

A diagram is not a smaller photograph. In our design exercise, students tried to show how a folding chair opened. Their first drawings included the grain of the wood, the pattern on the seat and the shape of the maker's badge. The drawings looked careful, but readers could not tell which joint moved first.

We covered the surface details and traced only the frame. A curved arrow showed the direction of the first movement. A second panel showed the chair after that movement had been completed. Nothing had been added to the chair itself. The drawing had become more useful by becoming less complete.

This does not mean that decoration is always irrelevant. A picture for a catalogue might need to show colour and texture. A drawing intended to help someone identify a damaged part might need another kind of detail. The question is not how much can fit on the page, but what the reader needs to do next.

A good diagram therefore makes a choice on the reader's behalf. It brings one relationship forward and asks other information to wait. That choice should be deliberate. An omitted detail can clarify a process, but an omitted warning can make the same diagram misleading.

How does the writer explain that useful diagrams depend on selecting the right details?