Workshop: Repairing the Ribs on a Neuner and Hornsteiner Double Bass 

Yup I have the same question

It might even be paper. Both materials are used by luthiers inside an instrument that has side grain, long and thin connections of wood. If you look inside a Napolitan mandolin for example, you'll see stripes of paper or cloth along the connections. It protects the glue and wood from air and moisture (from the outside varnish does this job) and gives a single thing for both parts to grab on (like the wood cleats but lighter and easier), increasing the structural strength of the body.