Updated May 21, 2021.
You probably think the word “diaper” refers to something babies wear, but it doesn’t. They are called diapers because the fabric often used for them had diaper patterns woven into it.
Diaper patterns are geometric patterns which have regular repeats in both diagonal directions. They also generally have regular repeats horizontally and vertically as well. But the diagonal repeat is what is critical here. So a stripe isn’t a diaper pattern (no diagonal repeat), but a check might be.
Many kinds of stitches, think of a block of Scotch Stitches, are diapered by nature. But in order to be a diaper pattern, there needs to become contrast in color as well. A background of Scotch in one color is not a diaper, but coloring the same block of stitches to look like gingham fabric does make a diaper.
The repetition is, I think, what makes diapers so lovely and compelling. Most diaper patterns are small so that you can really see the repeat. I think the prettiest ones alternate two or more different repeats in a regular pattern.
In general, diapers are usually small patterns and are abstract (though they don’t have to be).
Once you learn to recognize diaper patterns, you’ll start seeing them everywhere.
Oh and the baby diapers, the fabric often used for them was a pattern called Birds-eye, which has diamond units repeated all over. In fact, when my kids were little 20+ years ago you could still buy cloth diapers with this pattern. Since it’s been a while since I needed to do this, I don’t know if they still do.