The pens are made of 6061 aluminum, precision-machined and hard anodized. The heavy cap is threaded so that it will not pull off in use, and castellated for greater impact. The flutes and grooves on the pen, combined with the pewter-toned stainless steel pocket clip, provide a secure grip. Four color schemes are available: non reflective olive drab and tactical black, plus brown with bright grooves and black with bright grooves. All Tao models are functional writing instruments which use the Fisher Space Pen ball point cartridge, a pressurized design developed for NASA. It has a tungsten carbide ball point and thixotropic ink which will write in freezing cold, boiling heat, underwater and at any angle - even upside down.