The paper describes how designers used the construct of learning trajectories to create a tool, Math-Mapper 6-8, to help scaffold curricula toward increased learner-centered coherence. It defines "learner-centered curricular coherence" as "an organizational means to promote a high likelihood that each learner traverses one of many possible paths to understand target disciplinary ideas in a curriculum." The tool's features, including its learning map, diagnostic assessments, and reporting system, are tied to its underlying foundation in learning trajectories. Three preliminary studies of the implemented tool's effects are reported to provide insight to its influence on curricular sequencing, students' patterns of performance in early algebra, and student responses to the assessments.