This paper presents fundamental information necessary to understand and appreciate the concept of total gas energy in a natural gas pipeline. That is, to be able to converse with peers within the natural gas industry and understand basic concepts and terminology. The historical transition from volumetric measurement to total gas energy is discussed, including some of the basic terminology, physics, measurement, as well as the reasons for changes in methodologies. Included is industry acceptance of new concepts and regulations involving custody transfer as well as the instrumentation and systems involved in traditional and newer, more progressive forms of gas measurement