# 2 What is a diagram?

Diagrams are all around us. We all try to make sense of the world around us and this sense is displayed in two ways.

1. We all have our own ‘internal models’ of how things work based on our experiences and our interpretation of those experiences. These ‘internal models’ shape our thoughts and actions and lead us to expect certain outcomes from certain activities. They change and evolve with new experiences or (hopefully) when challenged by new information. They are the means by which we make sense of the world by searching for familiar patterns or creating new ones out of all the information we receive.
2. Although ‘internal models’ are personal we also use them to share our ideas and understandings by comparing them with those held by others through the conversations we have and the things we do. This enables us to build up ‘external models’, where patterns of ideas, things or activities are recognised and understood by many people at the same time, e.g. a table of the physical properties of matter.

   Such ‘external models’ underpinned by shared assumptions are commonplace, often to the point where we take them for granted. This can be particularly so where we try to use diagrams to represent and communicate complex information simply and quickly.

## Activity 1

Try to think of three examples of diagrams you see regularly in your home.

Write down what they represent to you.

Say what you use them for.

### Discussion

### Here are my responses

1. The first thing I had to hand was a ‘do it yourself’ (DIY) manual (being used a lot because I have just moved into a new house). This was full of diagrams of bits of rooms, fixtures, fittings, tools and other equipment that I might want to build or use myself around the house.

2. Next I picked up a road map from the hall. This is a very colourful diagram that uses many types of symbol, but I find it easy to follow, using the key, and invaluable in planning my car journeys. It ‘represents’ how certain features in the real world relate to each other in terms of distance. So I know that the distance from Milton Keynes to Birmingham is about 110 kilometres and that I can drive there in just over an hour (given the right traffic conditions!).

3. The last diagram I noted was the symbols on my wristwatch. This has a mixture of thick and thin lines around the outside of a circular face, but no numbers. I know from experience these lines represent the hours in half a day and minutes in a full hour and help me to ‘tell’ the time.

I can see a pattern in the examples I used in my answers. I started off with diagrams of real world objects albeit displayed in a way that makes things clearer than just a photograph of the finished item. Then I chose a road map, which is a very simplified diagram of what I would see if I were looking down from a plane. This diagram sets out to highlight certain things – such as roads, service-stations and motorway junctions – that are relevant to car journeys. Yet although the diagram shows things that are actually there for my eyes to see, it does not aim to look at all like what my eyes would see. It is symbolic and much easier to ‘read’ than an aerial photograph, because it leaves out a lot of unnecessary detail and has standard symbols for representing the things I need to know about. Finally, I ended up with a diagram of something you can't actually see, though it is a model of something you ‘know’ exists – time, represented by a sequence of minutes and hours. This last example is a more abstract representation of ‘reality’ than the earlier ones, but is still an efficient way of organising information.

Another impression I get from these examples is that they are all reasonably familiar. We all ‘understand’ and ‘use’ certain types and styles of diagrams from an early age, e.g. reading the time from a watch face. However the types and uses of diagrams are often rooted in different cultural, social or academic settings. Thus a modern map looks quite different to an eighteenth century map while digital watches have replaced analogue ones for many people.

OpenLearn - Systems diagramming
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