Let's revisit the graphic of the communication triangle to make it more complex. Imagine that the writer's purpose adds a layer over the top of the original triangle, and that the reader's purpose adds another layer:
But let's not forget that these elements also interact. As a writer, if I know why you're reading, that affects why I write (my purpose), and it should also help shape what I include or cut from the text. In other words, what I know about my purpose and the reader's purpose affects my topic choices. Anything else I know about a target reader also affects what I include in a text. Similarly, what readers know about a writer can affect how they construct meaning from the writer's texts. So each of the elements in our graphic so far interact to affect the message that finally becomes the finished text.