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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?

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    $\begingroup$ Isn't this pretty clear by looking at the picture? The diagram in the right hand side consists of two squares above each other. Then the top vertical arrows probably refer to the vertical arrows of the upper square and bottom vertical arrows refers to the vertical arrows of the lower square. $\endgroup$ – Jan Bohr Aug 4 '18 at 11:11
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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.

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