Each of the hollow hexagon beads is 17.5 mm between parallel sides, so printing two offset by 30 degrees lets you check whether your printer can accurately produce those dimensions in the purely X & Y directions. 

The hollow hexagon beads also have a hole that is 7.5 mm between parallel sides, allowing you to check to see which of the solid beads can pass through the hole.

Each solid hexagon bead is 0.05 mm smaller between parallel sides than the next, which is indicated by the number of chamfers on the top & bottom faces (1 chamfer = 7.495 mm, 2 chamfers = 7.490 mm, and so on)

All of the beads have a hole through them so they can be threaded onto a thick string or piece of cord. In the cover image, they are threaded onto the strings of my favorite hoodie.

Since these pieces are all separate, you can print this tester to check the tolerance of multiple variants of filaments, and then mix & match the beads for a wider range of colors!