Instructional research in German-speaking countries has conceptualized teaching quality recently according to three generic dimensions, namely, classroom management, student support and cognitive activation. However, as these dimensions are mainly regarded as generic, subject-specific aspects of mathematics instruction, e.g., the mathematical depth of argumentation or the adequacy of concept introductions, are not covered in depth. Therefore, a new instrument for the analysis of instructional quality was developed, which extended this three-dimensional framework by relevant subject-specific aspects of instructional quality. In this paper, the newly developed observational protocol is applied to three videotaped mathematics lessons from the NCTE video library of Harvard University to explore strengths and weaknesses of this instrument, and to examine in more detail how the instrument works in practice. Therefore, we used a mixed-methods design to extend the quantitative observer ratings, which enable high inference, by methods from qualitative content analysis. The results suggest the conclusion that the framework differentiates well between the lessons under a subject-specific perspective.