I was having difficulty finding t-nuts for m3 screws, especially for 20x20 extrusion. These printed t-nuts will work well enough, print quickly, and use very little plastic. You could probably print hundreds in to time it took to have some metal ones shipped to you. They are basically like flat washers that prevent the m3 nut from spinning or slipping through the channel opening. If you want a stronger solution, use m3 fender washers in conjunction with this part. I recommend printing the trunk of the T on the part itself, which will align the part with the channel and keep the holes centeredI popped the m3 nuts into the cavity on the plastic just after it finished printing, hoping it would contract onto the nut. The cavity is tight, at least on my printer, though the designed size is actually overly large. keep in mind that trying to tighten the screw too tight will strip the t-nut.