This study explored the relationships between task beliefs about the empty number line (ENL), mathematical ability, gender, and voluntary ENL use in multi-digit subtraction and addition. One hundred twenty-three Dutch third-grade students and nine teachers from six schools participated in this study. The multilevel path analysis showed that task beliefs about the ENL mediated the relationship between students' mathematical ability and their voluntary ENL use. No gender differences were found in the multilevel path analysis. Finally, the results show that task beliefs about the ENL and voluntary ENL use differed across classrooms. The discussion focuses on the implications of the results for using the ENL in diagnostic assessment.