When, a few years ago, I came across the drawing below in an article called ‘Line and borders of surfaces: Grouping and foreshortening’ by  John M. Kennedy, Igor Juricevic & Juan Bai in Looking Into Pictures. 321–354 (2003), I was dumbfounded. I had been drawing pictures for over 40 years without noticing that the lines I was drawing referred differently. Did I miss that class? You would have thought that an artist when making a line drawing would be aware that the marks he/she makes refer to different kinds of edges and boundaries. If Schier’s ‘natural recognition trigger’ theory of depiction is correct, that means I have been drawing lines whilst unaware what the lines are referring to and which lines will do the heavy-lifting work and kick-start the recognition of objects when someone looks at my picture. Imagine composing a sentence without any idea what each word referred to or how it operated in the sentence. Hmmm!