Wall plug aka screw anchor aka dowel to be used with attachment of a screw in porous or brittle materials. There are many forms of wall plug, but this one fits best with a concrete wall. The original design is rather small size, as that was the size i needed. EDIT: Version 2 of the plug has thinner walls and the gap between the two halves is larger. Works much better. This one is also easier to print. Version 3 has the separate two sides connected with a small bridge making it easier to print without the parts falling over. I suggest using a large brim when printing.