LPI
Lines per inch: the frequency of the halftone screen used in commercial printing.
Formula / rule
ppi ≈ 2 × lpi
Measured in
lpi
In depth
Screen ruling determines how fine the halftone dot pattern is, and it is limited by the paper: newsprint absorbs ink and cannot hold more than about 85 lpi, while coated stock supports 175 lpi or more. The traditional rule is to supply image files at roughly twice the screen ruling in ppi.
Examples in practice
A magazine printed at 150 lpi wants images supplied at about 300 ppi.