Introduction

Documents can contain the following types of figures: photographs, screen captures, scaled drawings (for example, plans, cross sections, and 3D views), and diagrams. With the exception of diagrams, they all show a representation of something that exists in reality, or will exist. A diagram, on the other hand, shows something that does not exist physically; for example, statistical data, or concepts that exists only in the human mind.

To show statistical data, we can use standard diagrams such as pie charts and graphs. This article does not describe methods for showing statistical data. Instead, it looks only at creating diagrams that show concepts; for example, sequences and relationships.

I suppose it's possible to argue that concepts do in fact exist — as mental pictures. In which case, to create an effective diagram, the skill is to put the mental picture onto paper or screen so that the reader obtains exactly the same information.

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What diagrams can show

This section lists ten types of information that diagrams can communicate. Often, a diagram conveys more than a single type of information. I have derived this list only by analysing how I have used diagrams in my own work. Hence, I certainly don't claim that the list is complete.

Diagrams can show:

• what something is. For example, during a seismic survey, sound waves travel through the earth's crust. The sound can travel along different path types, each having a specific name depending on how many times the sound is reflected and where the reflections take place. For each path type, a diagram can show where the sound travels.
• what something does. For a computer program that reformats data, a diagram can show the structure of the input data and the output data.
• the context. In a guide for a product that is used in only some departments in a company, a diagram can show the complete structure of the company and show which departments use the product and for what purpose.
• an overview. In an early chapter of a product guide, a diagram can show all the main functions or components of the product, often with cross-references to where the topics are described in detail.
• a picture of the words. This use of a diagram is appropriate for spatial information — both 2-dimensional and 3-dimensional. Even if the reader can get all the information from the words, the diagram provides reassurance and also makes it easier to recap the information.
• components. A computer model of a road network comprises several components. In simple terms, there are several types of point and several types of line. A diagram can easily show these components.
• comparisons. This use of a diagram is very helpful when there are several things that are fundamentally the same but have subtle differences. For example, the sound-wave paths I mentioned earlier in what something is.
• relationships. A diagram that shows components is also likely to show the relationships between the components. A computer program often uses several files; a diagram can easily indicate, for example, input files and output files.
• sequences. This is a common use of diagrams and it enables authors to show the correct order for a sequence of events or tasks.
• which item to select. This type of diagram steers the reader through the diagram to one of several end points. The diagram contains questions in decision boxes. At the first box, the reader selects one of the box's answers and then follows the corresponding arrow, which leads to the next question, and so on.

It's worth repeating that many diagrams will convey more than one type of information. Most of mine do.

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Secondary benefits for readers

Although the main reason for using diagrams is to communicate information, there are other benefits for readers because diagrams also do the following:

• make documents more appealing. This means readers are more likely to use the document in the first place. Moreover, they will maintain interest and receptivity longer.
• provide anchorages. In reference material, a topic might take several hours to read. This reading is unlikely to be continuous; diagrams make it easier for readers to pick up the thread again after a break.
• help navigation. Readers often remember diagrams; for paper documents, it's easy to flick through a document and find a diagram. Hence, particularly when a reader is familiar with a document, this method can be quicker than using an index.

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Benefits for authors

Using diagrams also provides benefits for authors:

• highlight gaps in author's knowledge. On many occasions, I have thought that I fully understood something and then, as soon as I tried to produce a diagram, I found that there were gaps in my knowledge about the topic.
• help research. I create diagrams while researching a topic. I often find it's essential to use these diagrams as the focal points of discussions with experts. It therefore seems obvious that I should include these diagrams in the document — if they help me to learn about the topic, they will also help my readers.

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