In the nineteenth century, Russian prisoners used a linguistic code to communicate through knocking on prison walls. Prison architecture responded to this communication possibility:

In the solitary cells of the Russian fortress of Saint Peter and Saint Paul, the floors are covered with painted felt. The walls are also covered with felt, and at a distance of five inches from the wall is a wire netting covered with linen and with yellow paper. The object of this contrivance is to prevent communication between prisoners by knocking on the wall.^