For structural use solid timber has to be strength graded. In order to stay economic, the grading process usually focuses on the most important physical and mechanical properties: density, bending strength (MOR) and modulus of elasticity (MOE). Grading standards also state how fürther mechanical properties can be derived from these basic property values by empirical relationships. The aim of this study was to review some of these property relationships as a contribution to a fürther development of strength class systems and grading standards. Based on mechanical tests of Norway spruce structural timber with different cross-sections, the following topics were evaluated with respect to the European strength class system (EN 338): a) simultaneous matching of basic values, characteristic values of density, MOR and mean MOE parallel to the grain, b) relationship of characteristic values of tension and compression strength parallel to the grain with respect to the corresponding characteristic value of MOR, c) ratio of 5^th- percentiles and mean value of density and MOE, and d) ratio of MOE in bending, compression and tension. With regard to machine strength grading, the correlation of (flexural) MOE and MOR as well as the potential of dynamic MOE derived from bulk density and ultrasonic wave speed as an indicating property for strength were evaluated. Over all the test results confirmed the property relationships and strength class profiles regarding MOR, tension strength, compression strength, density and MOE given in EN 338.