This study summarizes twelve years of research on the structure of forest stands of different tree species, focusing on the relationship between diameter distributions (thickness structure) and height–diameter curves (height structure). Based on numerous sample plots, the analysis compares distribution curves of the number of trees by diameter classes relative to the mean diameter (dav), and corresponding average height curves. Two newly introduced indicators – the asymmetry index (Ias) for thickness structure and the steepness index (Ist) for height structure – provide simple and practical tools for analyzing the dynamics of stand structure. Results demonstrate that all studied species (spruce, fir, Scots pine, seed-origin beech, seed-origin oak, coppice oak, and Scots pine plantations) show right-skewed asymmetry in diameter distribution, which corresponds to steeper height curves. Comparisons with Tyurin’s Unified Curve (1938) confirm the validity of these findings. The relationship between asymmetry and steepness makes it possible to evaluate stand productivity and wood volume potential more effectively.