As stamp printing methods progressed, so did the type of flaws that appeared on stamps. In photogravure printing the most noticeable flaws occur when specks of dust or grit becomes lodged between the rolling ink cylinder and the doctor blade. This causes the most of the ink to be drawn out of the recess, which then print only faintly whilst the dust or grit exists. If there is a fault in the blade, then ink is retained on the surface of the cylinder and prints as a thin line of colour.

As the printing continues, the doctor blade travels transversely across the revolving cylinder, which in stamps printed in an upright motion, will produce a solid vertical line. Likewise, stamps printed with the design sideways will produce a horizontal line. These lines will always have a diagonal direction although it will probably not be noticeable. As the printing continues it is likely that the doctor blade flaw will continue to all stamps in the sheet, whereby the diagonal angle will become clear.

Many stamp experts, who collect doctor blade flaws, often try to find similar stamps that continue the line. It is of much interest because the cause of the flaw is always clear, and the flaw can only occur on stamps printed using rotary photogravure.

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