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Since the “vanadium trimers” in layered LiVO₂ has been proposed by Goodenough in 1963, many experimental studies have been carried out to clarify the realization of the trimers. Our comprehensive structural studies using synchrotron X-ray diffraction experiments clarified that vanadium trimers are randomly aligned along the c-axis direction in LiVO₂, while the long-range ordering of vanadium trimers along the c-axis direction appears in LiVS₂, which is a sulfide analog of LiVO₂. In this presentation, we will explain that the presence/absence of such a periodicity in the trimer patterns is brought by the difference in their inherent stacking structures. We also present that the trimerization of LiVO₂ without a long-range ordering along the c-axis direction, can be identified using pair distribution function (PDF) analysis based on appropriate structural models.