Which term describes the formation of regular light and dark patches by interference of two halftone screens?

• A. Isogonic
• B. Moire
• C. Grid
• D. Isopleths

Answer: (B)

Moire patterns appear when two regular dot grids overlap, creating a new pattern of light and dark bands due to the interference of the two periodic structures. In halftone printing, tones are reproduced with dot screens at specific angles and frequencies. When two halftone screens overlay—especially if their frequencies are similar or their angles are not sufficiently different—the overlapping dots produce a larger, repeating pattern of lighter and darker patches. This phenomenon is what the term moire describes.

Other terms don’t capture this interference effect: isogonic lines relate to magnetic variation, isopleths are contour or equal-value lines, and a grid is just the underlying lattice, not the emergent interference pattern caused by overlaying two screens.