Two longitudinal extensions of the classic internal/external frame of reference model (I/EM) have attracted researchers' attention in recent years: The reciprocal I/EM (RI/EM) describes the reciprocal effects between students' math and verbal achievements and self-concepts. The 2I/EM describes the effects of students' math and verbal achievement levels and changes on their math and verbal self-concepts. The present research integrates these 2 approaches into the reciprocal 2I/EM (R2I/EM), which describes the effects of students' math and verbal achievement levels and changes on their math and verbal self-concepts and the effects of their math and verbal self-concept levels and changes on their math and verbal achievements. We tested this model in 3 empirical studies, including about 6,000 students between Grade 4 and Grade 11 from Germany, Switzerland, and Belgium, using grades and test scores as achievement indicators. Overall, we found strong support for the R2I/EM. In line with prior 2I/EM studies, students' achievement levels showed positive (negative) effects on their self-concepts within (between) subjects. The effects of students' achievement changes on their self-concepts within (between) subjects were usually positive (near 0). In line with prior RI/EM studies, students' self-concept levels showed positive (negative or near 0) effects on their achievements within (between) subjects. For the first time, we also examined the effects of students' self-concept changes on their achievements: These effects were usually positive (positive or near 0) within (between) subjects. Our findings have important theoretical and practical implications for the I/E model literature and for teachers aiming to foster their students' self-concepts and achievements. Educational Impact and Implications Statement Previous research has shown that students' subject-specific self-concepts are formed by the joint operation of three comparison processes: social comparisons (where students compare their achievements with their classmates' achievements), dimensional comparisons (where students compare their own achievements in different subjects), and temporal comparisons (where students compare their own achievements across time). The findings of the present research suggest that students conduct comparisons not only of their subject-specific achievements, but also of their subject-specific self-concepts, with resulting effects on their subsequent achievements: In particular, we found positive effects of changes in students' self-concept in one subject (i.e., temporal comparisons of self-concepts) on their subsequent achievements in the same and other subjects. This finding is of particular practical importance, as it demonstrates that teachers can help students enhance their achievements in different subjects via impacting on their self-perceptions (i.e., fostering students' self-concepts and supporting students to become aware of increases in their self-concepts).