Isomorphic Bookmarks (for books)

Most bookmarks simply won't do. They let you know roughly which page you are on (but is it the left page or the right?), yet they don't tell you which line you stopped at. Here I will describe how to construct, out of ordinary household materials, an isomorphic bookmark which uniquely identifies the line on which you stopped reading.

The construction of an isometric bookmark is straightforward:

• take an index card and cut it in half
• draw a line from the center to one of the corners, as illustrated below

Fig. 1. Isometric bookmark

The isometric bookmark has the following property: For any line L on page P, there exists an orientation θ that uniquely identifies L on P.

In other words, by turning the card you can have it point at any line on any page – left-facing or right-facing, near the top, middle, or bottom, as shown below.


Fig. 2. Bookmarking a line near the top of a left-facing page.

There are line-based bookmarks like LastLine that you can buy at bookstores, but it's hard to beat the isomorphic bookmark for simplicity, cost, and structure-preserving topological invariance.