Amorphous carbon is free, reactive carbon that does not have any crystalline structure (also called diamond-like carbon). Amorphous carbon materials may be stabilized by terminating dangling-p bonds with hydrogen. These materials are then called hydrogenated amorphous carbon. As with all amorphous solids, some short-range order can be observed. Amorphous carbon is often abbreviated to aC for general amorphous carbon, aC:H or HAC for hydrogenated amorphous carbon, or to ta-C for tetrahedral amorphous carbon.