In traditional typography, a counterpunch is a type of punch used in the cutting of other punches, usually to create the negative space in or around a character. Such a tool solved two issues, one technical and one aesthetic, that arose in punch-cutting.
Punches and punch-cutting
In the digital world of typeface design, we tend to think of characters as two-dimensional objects. This is very different from the way that early printers thought of them. The first step in creating a character for use in typesetting was cutting a steel punch. A mold could then be created from the punch by using the punch on a softer metal (such as copper). Finally, from this mold, plates for printing could be cast.
How far into the mold, and at what angle, a punch was hammered could significantly affect the appearance of the character when it was printed. This also depended on the three-dimensional shape of the punch. Creating a punch therefore depended on consideration of more than just the two-dimensional shape of its striking surface, and particularly of the slope of the sides of the punch.
The technical issue
Punches were small, usually on the order of a few millimeters, and cutting them at all was a difficult task that required much practice. Particularly difficult was cutting out the center of a letter (e.g. lowercase "a"), cutting deep enough, and cutting straight into the metal, rather than at a slope (remember that these punches were made of steel). In fact, this task was so difficult that it was almost never done with cutting tools. This is where the counterpunch comes in. A counterpunch could be used to create this negative space, not just where the space was completely enclosed by the letter, but in any concavity (e.g. above and below the midbar in uppercase "H").
Of course, the counterpunch had to be harder than the punch itself. This was accomplished by heat tempering the counterpunch and softening the punch.
The aesthetic issue
Often the same counterpunch could be used for several letters in a typeface. For example, the negative space inside an uppercase "P" and "R" is usually very similar, and with the use of a counterpunch, they could be nearly identical. Counterpunches were regularly used in this way to give typefaces a more consistent look.
