Impression block:

A block with lead or another relatively soft material on its bottom.

It is made up of drill pipe or tubing at the surface, run into a well, and set down on the object that has been lost in the well.

The block is retrieved and the impression is examined.

The impression is a mirror image of the top of the fish and indicates the fish’s position in the hole, i.e., whether it is centered or off to one side.

From this information, the correct fishing tool can be selected.