Foundations in Graphic Communications: Checking Color Proofs  
Conventional Screening
Every halftone image consists of overprinted screens, one per color to be reproduced.
The overprinted dots form regular patterns called rosettes.
To form the rosettes properly and reproduce color accurately, the alignment of the overprints is critical.

cmyk angles
moire pattern
10%, 50% & 78% dots
During imaging, the screens are set at specified angles from one another so that the proper overprinting will be achieved.
Incorrect screen angles can produce a printing defect know as a moiré pattern, or simply moiré.
In conventional halftone screening, the sizes of the dots vary, but the spacing between them does not. The distance from the center of one dot to the center of an adjacent dot is always the same, regardless of size or location. The size of the dot depends on the density of the image area in which the dot occurs. Highlights contain small dots; midtones, medium-sized dots; and shadows, large dots.