A unit of measurement specifically describing the linear number of halftone dots at a particular dimension (in this case, inches.) 

LPI applies exclusively to print materials on standard printing presses. It does not apply to photographic prints, ink jet prints, or digital display on TVs, tablets, or phones.

Most book printing uses a halftone dot pattern (“Screen”) containing 150 LPI, while screens of 175 LPI and 200 LPI are not uncommon. 

LPI can also be described as “Course” (65 LPI used commonly for lower print-quality news papers) and “Fine” (200 LPI) used primarily for art reproduction and high quality books and magazines.

Publications printed using a fine line screen can reproduce detail more clearly than a course screen. A fine line screen (LPI), when used to reproduce a low-resolution image (PPI) can reveal the artifacts contained in the original image.

The dot pattern in printing can be seen by the naked eye or using a simple magnifying glass.

Frequently confused with Pixels Per Inch.