A four-color process job has at minimum four plates: cyan, magenta, yellow, black. A label with two spot colors plus white underbase has three plates. Each plate prints separately, the substrate passes under each station in sequence, and every plate has to put its ink down in exactly the position the file specified.
"In register" means all the plates agree on where the artwork sits. "Out of register" means one or more plates is offset by some fraction of a millimeter. The offset can be horizontal, vertical, rotational, or all three. The result is visible misalignment where colors touch.
White slivers between adjacent colors. A red shape next to a blue shape, with no overlap designed into the file, will show a thin white gap on whichever side the registration drifted. The eye reads it as a misprint immediately.
Double-vision text. Small black text printed over a colored background sometimes carries a knockout of itself on the colored plate. If the black plate drifts, you see the black character with a colored halo on one side and a colored ghost on the other.
Smeared logo edges. A logo with a thin colored outline around a different colored fill will show the outline thickening on one side and disappearing on the other.
Color shifts in fine detail. Small process-color images can lose sharpness because the four plates that combine to make the photographic image aren't lining up. Magenta drifts left, yellow drifts down, and what looked sharp in the file looks soft on press.
| Press | Typical tolerance | Trapping needed? |
|---|---|---|
| Offset (sheetfed) | 0.1mm (0.3pt) | Optional on critical edges |
| Offset (web) | 0.15mm (0.4pt) | Recommended on adjacent colors |
| Flexography | 0.2–0.3mm (0.6–0.85pt) | Required on adjacent colors |
| Digital (HP Indigo, Xeikon) | 0.1mm (0.3pt) | Rarely needed |
| Screen printing | 0.5mm (1.4pt) | Required, generous overlap |
Digital presses image directly without physical plates, so registration is essentially handled in software. Drift is rare and small. Flexo has the most variance because the plate is rubber-soft and the web stretches under tension.
Registration marks are small targets, usually a crosshair inside a circle, placed outside the trim area on every plate. The press operator (or the press's automated camera system) uses them to confirm that all plates are landing on top of each other. They are not printed on the final piece; they live in the bleed margin and get trimmed away.
In Illustrator, registration marks are usually applied as a print preset rather than drawn manually. The marks must be in 100% of every plate (this is what the "Registration" color swatch does) so they appear identically on every separation.
Trapping is a small, deliberate overlap between adjacent colors that absorbs registration drift. If a red shape borders a blue shape, you slightly thicken the red so it laps onto the blue (or vice versa) by 0.1–0.3pt depending on the press tolerance. When the plates drift, the overlap closes the gap before it becomes visible.
Modern RIPs perform automatic trapping, so most files do not need designer-applied traps. But the file has to be set up correctly. Overprint flags must be honest, knockouts must be intentional, and rich black should be specified deliberately. When a preflight flags trapping issues, it's usually because something in the file is going to confuse the RIP's automatic trap engine.
Preflight flags overprint mistakes, missing trap allowances, and the small file-prep errors that turn into white slivers on press.
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