The optimal way of folding a bond notebook page into a bookmark

The article explores a simple yet interesting geometric problem: how to fold a notebook page so it is the most visible when the notebook is closed (i.e., as a bookmark). We start with a square page, then proceed to the rectangular page. When an additional constraint is added to limit the vertical extent of the rectangular page, the problem shows the first-order phase transition: the optimal way of folding changes drastically when the aspect ratio of the page exceeds the critical value of about $1.20711$.