Here in her short note in the middle of the page 1, Emily Riehl says ...with the top two vertical arrows ... I just wonder how vertical arrows can be top or bottom. I would say that vertical arrows may be either left or right and on the contrary horizontal may be top or bottom,right? What does she mean by top two vertical arrows?
In the right side picture we have four vertical arrows, two of them are top arrows and the other two are bottom arrows. You also have three horizontal arrows and you may talk about top two or bottom two horizontal arrows.