The correct term for these pieces is "fragmentation" (sometimes shortened to frag); "shards" or "splinters" can be used for non-preformed fragments. Preformed fragments can be of various shapes (spheres, cubes, rods, etc.) and sizes, and are normally held rigidly within some form of matrix or body until the high explosive (HE) filling is detonated. The resulting high-velocity fragments produced by either method are the main lethal mechanisms of these weapons, rather than the heat or overpressure caused by detonation, although offensive grenades are often constructed without a frag matrix.