Printing presses are unable to deposit ink partially. They can either make an area of the page black (or whatever color the ink is) or white, but nothing in between. You can think of a press as a giant rubber stamp.

In order to simulate ink being mixed in different quantities, small, evenly spaced dots are used. The more ink required in an area, the larger the dots are. This is known as halftoning. These patterns of dots are known as halftone screens and vary in density for different amounts of ink (from 0% to 100%). A gradient printed with a halftone screen is shown below.

An enlargement of a halftone gradient.

When several inks are used (as in four-color process printing) a halftone screen is used for each color. Inks are laid down lighest to darkest, and each screen is set at a different angle relative to one another. If two screens are set at angles too similar to one another a distracting moire pattern can result. There are many different standards for halftone angles, but Y=0°, C=72°, M=18°, and K=45° work well together. If only one ink is being printed (such as for a black and white photo) an angle of 45 degrees is used. Well-chosen screen angles create small, circular "rosettes."

If screen angles aren't chosen wisely they can cause interference patterns with one another, as shown above. This is known as the moire effect.

The illustration below shows the four halftone screens used to print a CMYK image.

The halftone screen for each ink is set at a different angle. When combined they form the full-color image.

Most consumer color printers do not use or require halftone patterns. Instead, they spray tiny dots of ink. Dots are sprayed in greater or lesser quantity depending on the color being printed.

Sample image: detail of "Portait of Henri Gasquet," painted by Paul Cézanne, c. 1896–97.