and rack, the bed, carrying with it the die, is moved back and forth
under the roll. This is called "rocking" and by it the soft steel of the
roll is forced into the die and a reverse impression of the design is
obtained. The roll is then hardened and, by a reversal of the process,
impressions from it are transferred to the steel plate from which the
stamps are to be printed. The plate is, of course, soft at first and is
hardened after the required number of designs have been transferred to
it. This process is so perfect that the most delicate lines of the die
are repeated with absolute fidelity on the plate. When many plates of a
stamp are likely to be needed, it is customary, in order to avoid risk
of wear or damage to the original die, to make duplicate dies, called
transfer dies, and from them the necessary rolls to make the plates.

The plates are made with great care. They are touched up by hand and
subjected to close scrutiny and the work is often gone over a number of
times before the result is pronounced satisfactory. Incidentally any
guide lines and marks used by the transferrer are removed by burnishing.
In the older issues of United States stamps, such lines and dots are
frequently found on the stamps but the later issues are very free from
them.

Plates that have become worn are "re-entered," that is to say, the
transfer roll is applied to the plate in the original position and the
lines thus sharpened and deepened. If, by any mistake in making or
re-entering a plate, the roll is incorrectly placed and then changed to
the correct spot, a double impression of some of the stronger lines will
result. This is called a "double transfer" and sometimes, though
wrongly, a "shifted die." These double transfers are quite common in the
United States stamps made before 1861 but are scarce in the late issues,
either because the work is now more carefully done or because any