This paper introduces the LIDO, or the Low-Inference Discourse Observation tool, that captures discourse moves produced by students and teachers in whole-classroom discussions. Coding methods are described, followed by analyses that explore validity of the LIDO through correlations among LIDO-coded discourse moves and between LIDO scores and scores on the Instructional Support domain of the Classroom Assessment Scoring System-Secondary, utilizing 643 audio-recorded classroom lessons. Observations were conducted in fourth through seventh grade urban classrooms, including English Language Arts, Mathematics, Science and Social Studies. Rates of teacher and student discourse moves correlated with each other and with CLASS scores in expected ways, providing evidence of internal and convergent validity. Implications for use in research are discussed, including specific advantages of this new approach, such as the capacity to tease apart teacher behavior from student behavior in the context of classroom interactions.