Most discussions of measurement theory are focused on "scales" of measurement, but it is not clear whether reference is made to the mechanisms of measurement or the metric information derived from measurement. This emphasis on scales in measurement theory has not always provided a meaningful or fruitful description of measurement activities in psychology. It is proposed that a new concept, that of a standard system of measurement, be introduced to refer to the mechanism of measurement. There are three types of standard systems: (1) elementary, using human judgment; (2) devised, consisting of tests and physical instruments; and (3) dual-process, or self-report instruments. A measurement theory based on the standard system concept provides for a strengthened rationale for measurement activities in psychology. (Author/BS)