You receive a print run, unpack the pallet, pick up the first sheet, and something catches your eye. The black text seems to slightly "drift" away from the background color. A thin white or colored line where there should have been a clean edge. You call the print shop to complain that "something went wrong," and on the other end you hear a calm reply: "that's normal with offset printing, it's a matter of registration fit." And that's when the client usually frowns, because no one told them beforehand that offset printing has its own physical limits of precision.
So before anyone thinks this is just an excuse for the printer's own shortcomings - no, it's not an excuse. It's a real phenomenon built into the technology, and it's worth knowing about before you design something that requires perfect color alignment down to a tenth of a millimeter.
What registration and color fit actually are
In offset printing, each CMYK color - cyan, magenta, yellow, and black - is applied separately, from a separate plate, in a separate pass through the press. The sheet of paper passes through four (or more, if spot Pantone colors are added) printing units, one after another, and each time it must land in exactly the same spot with an accuracy of a fraction of a millimeter. Registration is precisely this process of aligning the sheet against each successive plate. Color fit is the final result - whether these four colors actually landed on top of each other exactly as the designer planned.
Sounds simple on paper. In practice, it's a juggling act with a sheet flying through the machine at several meters per second, where the paper stretches and shrinks due to humidity, and the gripper mechanism itself has its own mechanical tolerance. Industrial-class offset presses (Heidelberg, KBA, Komori) can maintain registration within 0.05–0.1 mm - that's genuinely precise work. But zero doesn't exist. There's always some margin of error, even if minimal.
Why misalignment happens in the first place
Paper isn't glass. It's a hygroscopic material - it absorbs moisture from the air and releases it, depending on the humidity in the production hall, the temperature, and even how long the sheet sat in a stack before printing. A sheet that enters the press with slightly different moisture content than the previous one can stretch by tenths of a millimeter - and no operator can fix that manually on the fly, although of course the machine is constantly calibrating and correcting itself during operation.
On top of that there's the mechanics of sheet gripping itself - the grippers on the cylinder must catch the paper at exactly the same point every time, for every color. With a run of 10,000 sheets printed in four colors, that's 40,000 individual "grips." Statistically, some of them won't be perfect. Add to that the stretching of the offset blanket, temperature changes in the hall throughout the workday, minor fluctuations in ink tension - and you start to understand that perfect 1:1 fit isn't the default state of things, it's the result of the machine's constant battle with physics.
A real-life example
Take a simple business card with black text on a colored background - say, white letters on a deep navy background made of C=100 M=80 Y=0 K=20. If the plate carrying the black channel (K) shifts relative to the other three by even 0.15 mm, you'll see a thin sliver of color along the edges of the letters - blue or purple, depending on the direction of the shift. To the naked eye without a magnifying glass, this might be unnoticeable. But with large fonts, thin lines, or a precise graphic frame - yes, it's visible to the naked eye, and the client is understandably annoyed.
Trapping - how print shops cushion this problem
This is where a technique called trapping comes in (not to be confused with a "color trap" in the colloquial sense - this is a specific prepress technique). It involves deliberately, minimally extending one color underneath another at the point where they meet, so that any registration shift doesn't leave a white gap between objects. Instead of two colors meeting at a perfect edge-to-edge seam, one of them slightly "slides" under the other, for example by 0.1–0.15 mm.
This is usually done by the print shop itself during prepress (at the RIP stage or in imposition software), not by the designer in Illustrator - but there are situations where it's worth thinking about already at the design stage, especially with graphic elements featuring large color areas meeting thin lines or text in a different color.
Overprint versus knockout: If your design has black text on a colored background, make sure the black is set to "overprint" rather than "knockout." With overprint, the black prints on top of the colored background, so even with a slight registration shift you won't see a white line under the letters - because underneath there's still color, not a "hole" cut out for the text.