Some people find reading block text challenging, using a window guide can alleviate the problem. This bookmark is design designed to aid reading in that way. The top is thicker to keep the guide square to the edge of the book, with an indent on the top (bottom when in use) to make it easier to use. There are two sizes on separate plates to suit most paperback books. The models can be sized in the x or y dimension to suit the size and typographic design of your book. I've used a beige coloured filament, but some people might find a contrasting colour more suitable.