When a well is being drilled, exposed formations must be periodically covered and protected by steel pipe. This is done for several reasons — to keep the hole from caving in, to protect the formations being drilled and/or to isolate different geological zones from each other. These protective pipes are called casings and liners. Casing refers to pipe that starts at the surface or mud line and extends down into the borehole. The term liner applies to pipe whose upper end does not reach the surface or mud line but is inside and overlaps the bottom of the last casing or liner. Casing and liners are either totally or partially cemented in place.