The Conditions of Economic Progress that became an established component of the economic lexicon. In Economic Conditions, Clark describes a tripartite economic structure consisting

Primary, secondary, and tertiary economic activities. Primary activities are agricultural or extractive in nature and are limited by natural growth factors. Secondary activities are primarily composed of manufacturing and production activities and are limited by mechanical factors. Tertiary activities are service-based and are dependent on, and limited by, human skill and expertise. Primary and secondary activities can be further distinguished from tertiary activities by the nature of their output, which is tangible in nature, whereas tertiary activities produce intangible outcomes. The tertiary for Clark is also a repository for activities which do not fit under the narrower and much more traditional understandings of the first two sectors.