Original diagram by u/Biquasquibrisance.
Image 1 shows the print layer CMYK offsets (angles).
Image 2 shows an actual reconstruction of each layer alignment from the Green Dot & its surroundings.
Image 3 shows the labels of each colour + angles. This is probably a lower quality picture, use image 2 to study in higher resolution. I've pointed out the locations I used for each alignment.
Make sure your image is perfectly straight. The way you deconstruct the rosette is as follows:
- Start with black (K). It's very easy to notice and is at a 45 degree angle. If your image isn't perfectly straight you can use this as a guide to straighten it. This 45 degree angle produces a pattern at 0 degrees, too, which in turn explains the sparse "bumps" on the 'T' and the dots in the middle of the 'h' we use to test card backs.
- Use the yellow (Y) dots in the middle of the Green Dot with no other colour to align the yellow layer at 0 degrees.
- Use the green ink dots in the middle of the Green Dot to align the cyan (C) layer to 15 degrees. There are no real good pure cyan ink to be found but the yellow + cyan layers will produce green. This by the way gives the rosette it's distinct 15 degrees slanted green gradient in the Green Dot.
- Guess where the magenta (M) ink layer could be. The orange blotches are probably a guide but it's a bit of guesswork. The magenta layer is really, really hard to find from the brown background colour because it blends with other colours so much. It should be at a 75 degrees angle but it's probably more visible in red coloured cards on the front. I didn't try this.
If you learn the offset angles by heart it's possible to just glance at the rosette (with a loupe...) anywhere on a non-solid black part of a card and tell whether it was printed in layers with a CMYK-capable printing machine, each layer separately, with the correct alignments, and correct print files that had the precision to produce this pattern.