In many ways, research on linear measurement has not answered fundamental questions that were asked 2 decades ago (p 893). Research suggests that students construct meaningful understanding of length measurement as they abstract and reflect on the process of iterating unit lengths (p 895). In contrast to the Piagetian view, a simpler explanation of the mechanism for length conservation is possible. As children repeatedly compare abstracted scanning motions for non-aligned equal-length objects by placing them in appropriate end-to-end alignments, they abstract a general reversible alignment process. This process, coupled with exposure to adults who interpret the process as indicating that the length of an object does not change when it moves, builds a reasoning structure that supports conservation (p 897).