From Metalworking Wiki

Pein: also spelled "peen". This is the part of the hammer opposite of the flat striking face that is used for general forging. The pein is usually wedge shaped, or rounded(spherical). It is used to indent, shape, spread, or stretch the material being forged. The pein can be made at any angle to the body of the hammer, but usually are 180º (straight), 90º (cross), 45º (diagonal, can be left handed or right handed in orientation as well). The rounded pein is referred to as a "ball" pein.