Tangent ruler

This is a simple ruler which draws circles from two tangent points.

To use, simply hammer a couple of nails into your favourite table, then use a pen to trace out the circle while holding the ruler against both nails.

Voila, a perfect circle!

Inspired by this pic online: https://imgur.com/t/aiko/mu96ohu

I must try and remember this trick when doing metalworking, I can see it being useful for laying out shapes against existing features.