A short note on terminology is helpful at this point. We have spoken of models and introduced them formally. We have spoken of a sketch as a partial representation of a model, with the equations hidden except for basic causality.
A sketch is also a model in its own right. You can build a model without ever entering equations. Such a model can be very useful for communication, organization, etc., but cannot be used for simulation. These word-and-arrow models can be causal loop diagrams, stock and flow diagrams, or other visual models. If you think of the Sketch Editor as a tool for building word-and-arrow diagrams, you will find it is powerful, flexible, and consistent with a number of different modeling paradigms.
We will use the terms "word" and "variable" largely interchangeably in this chapter. Comments and junction nodes are "words" that are not "variables" since they do not form a structural part of a model.
