... Perforation Techniques ...

In the technique called line perforation (German: Linienzahnung) the horizontal and vertikal perforations are made separately. Often, this results in irregular perforation at the corners of the stamp, and sometimes even different stamp dimensions if the distance between perforation lines is not uniform over the complete sheet.

Fig. 1 Line perforation

Fig. 2 Fake 30 st Bulgaria (Mi 11)
with line perforation corner

In the technique called comb perforation (German: Kammzahnung) horizontal and vertikal perforations are made simultaneously. This results in symmetrical perforated corners of the stamp. There are several variants of the comb perforation techniques, depending on how many rows are perforated simultaneously. Harrow perforation (German: Kastenzahnung) is a similar technique where the complete sheet is perforated simultaneously.

Fig. 3 Comb perforation

Fig. 4 Original 30 st Bulgaria (Mi 11)
with symmetrical comb perforation corner