Threadless headsets are lighter than threaded headsets, easier to assemble/disassemble and cheaper to manufacture. They're pressed or placed into the head tube, ensure smooth steering and are divided into external (EC - External Cup), semi-integrated (ZS - Zero Stack) and integrated (IS - Integrated Standard). Furthermore, there are tapered threadless headsets for tapered fork shafts which are often used with ZS and IS variants. Here, the outer headset is pressed into the head tube and both bearing shells are visible from the outside. Semi-integrated headsets have both bearing shells inside the head tube; only the thicker ends are visible from the outside. Fully-integrated headsets aren't pressed into the head tube. Instead, both bearings are loosely inserted into the bearing shells, which simplifies installation and replacement.