Notching is an affect caused by the formation of a doubled die. The notch is formed on the serifs of letters and numerals and is the result of the positioning of the first image to the offset of the second image. While true notcing is only formed on doubled dies, a condition that is similar is seen with die deterioration doubling.

The image below shows notching on a 1964 Lincoln cent doubled die. The image is courtesy of Coppercoins.