A line pair can be more than one thing.
- A guide line pair has a guide line between the stamps. Since the guide lines are deliberately incised into the plate, they will generally be sharp and clear.
- A joint line pair has a joint line between the stamps, deriving from the seam in the cylindrical plate used to print the stamps. These lines are somewhat smeared in appearance.
In image processing
A line pair in image processing is this: on a test pattern consisting of closely spaced black parallel lines on a white background, a line pair is one black line together with the white space on one side of it. When photographers say "lines of resolution", they mean line pairs. In television and video, one line pair equals two TV lines or pixels, of resolution.