A Little Book of Almost Triangles

It arrived in a matchbox,
stitched in thread the colour of old milk.

The first page showed a triangle
with one side tucked under the binding.
The caption said, *Look beneath your thumb.*

I did.
A crease appeared between two vertices.

The next plate showed parallel lines
sharing one umbrella,
each convinced it was raining.

Then a circle drawn with a ruler.
Below it: *Centre withheld pending review.*

The last page showed my thumb,
at the exact angle that held the book open.

I shut the cover.
The crease across my thumb
held the book open.