Historically, some design elements near the outer perimeter of a die were added by punching because technology made it difficult to press both the central and perimeter design elements at the same time. Also, some dies were made without mint marks or dates to permit their use at different mints and in later years. A die technician added missing elements by positioning a punch, a small steel rod with the mirror image of a letter or number on it, and striking the punch with a hammer pressing the image into the die. If the image is not strong enough, the technician will punch it a second time. Punches placed in a different position between strikes will produce a doubled image which is called a re-punch. Dual punches occur when punching is repeated in a second location. Sometimes technicians use a punch with the wrong or incorrectly sized letter or number.