Capillary or solder joints
The fittings are made of slightly larger diameter copper which fits exactly over the pipe. Applying heat and running a band of solder into the ends of the joint makes a watertight connection. The solder is drawn into the gap by capillary action, hence the name. The use of this type of fitting has become more extensive with the introduction of connection pieces preloaded with solder. They are much less expensive than compression fittings and also far less obtrusive since they are almost the same size as the pipe itself.