Some reviewers say it's a hard plastic, but it's a comfortably soft, pliable rubber. I measured most of them and they were true to size. The first dimension is the inside diameter of the hole (for wire or whatever) and the second dimension is the size of the hole you need to drill for a tight fit. However, the thickness varies. and for most large plugs and grommets you only get 2 or 3 of each. But for the price this is a good set, especially for vehicle wiring. The plastic case is junk, one of the two mounting tabs broke off instantly, and the hinges don't last forever, but at least the dividers are strong and can fall out.