In photographic science, a Mackie line is an adjacency or border effect created during development, at the border between areas of high and low densities.

During developing developer remains relatively fresh in an area of low density as less developing takes place and consequently, developer oxidation product concentration remains relatively low. At the border between high and low density areas the relatively fresh developer diffuses

Diffusion

Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

 laterally into the high density area and causes there a continuation of development. The result is an increased border density of the high density area.

The Mackie line effect belongs to the family of edge effects. Edge effects include: Gelatin-, Ross-, Fringe-, Eberhard- and Kostinsky effects.