A generalized three-dimensional concept of continuous forest inventories applicable to all common forest sample designs is presented and discussed. The concept recognizes the forest through time as a three-dimensional population, two dimensions in land area and the third in time. The sample is selected from a finite three-dimensional partitioning of the population. The partitioning is analogous to carving the volume into pieces like a three-dimensional jigsaw puzzle. Each puzzle piece is defined by the selection volumes of observation sets on the individual trees existing in the forest during the period of interest. The concept is a temporal extension of an alternative view of forest sampling offered in Roesch et al. (1993, Sur. Methods 19(2):199–204) and results in a finite number of independently selected three-dimensional sample units.