A specification system for tube outside diameter and wall thickness peculiar to pressure tubing made of copper. In these systems, each nominal outside diameter (.125 of an inch smaller than the true value) is associated with a particular wall thickness. Type K tubing is heavier walled than Type L, and was originally intended to specify copper tubing for use in underground plumbing and heating systems. Fabricators of this type of tubing will frequently specify the diameter by the nominal value without reference to the fact that it is Type K or Type L tubing; therefore it is always prudent to double-check the diameter given for a copper tube.