These are Re-Caps for the S1 Pro Knee Pad and S1 Youth Pro Knee Pad.  They come in a set and are made of High Abrasion PC Plastic.  S1 Pro Re-Caps come equipped with high grade male /hook velcro so that you can easily install them to your knee pads. S1 Pro Re-Caps are compatible with the S1 Pro Knee Pads and S1 Youth Pro Knee Pad.