Attached to the south-southwestern outer rim of Tamm is the smaller van den Bos.

The interior floor is level and is marked only by tiny craterlets and a few clefts along the edges.

The floor has merged with the interior of van den Bos to the south.

It has been hypothesized that the fissured, viscous-appearing material within both Tamm and van den Bos was emplaced as impact melt from the Mendeleev basin 225 km to the northwest.

[1] By convention these features are identified on lunar maps by placing the letter on the side of the crater midpoint that is closest to Tamm.