This paper presents the results of a parallel examination on the strength of oval tenon-mortise joints in two groups of test joints. Joints from group A were made with both shoulders of tenon, while joints from group B were made without the lower shoulder. Examination of strength was conducted on a chair joints rear leg-rail. The results of the examination indicated that joints in group A (oval tenon with the upper and lower shoulder) had a higher average bending moment capacity in comparison with the joints in group B (oval tenon without the lower shoulder). However, the significance of difference in strength between the two groups of joints was not confirmed by the statistical analysis.