Briggs and Peck make a compelling case for creating new, more intuitive measures of learning, based on creating vertical scales using learning trajectories (LT) in place of "domain sampling." We believe that the importance of creating measurement scales that coordinate recognizable landmarks in learning trajectories with interval scales cannot be understated. As we have learned from the previous decades of high-stakes assessment, when tests are required to serve multiple purposes, such as improving systemic accountability and providing instructional guidance, one purpose often suffers. These high-stakes tests have provided weak instructional guidance, producing unintentional outcomes such as excessive focus on "bubble kids" without sufficient information on how to promote deep learning (e.g., Horn, Kane, & Wilson, 2015). And in spite of the field's best efforts, we are still without a set of measurement scales that communicate how different areas of the scales are related to qualitatively different states of children's coming to know big ideas. Our own current research program is devoted to a similar goal. Instead of end-of-year, large-scale testing, however, we target periodic intrayear diagnostic uses of such scales without high-stakes consequences for teachers or students. We are designing our measures to inform teachers and students about student thinking and learning. In this response to the Briggs and Peck paper, we wish to comment on 2 broad issues related to the paper: (1) the modeling theories that underlie the relationships between measures and the phenomena they are designed to index and (2) the details and challenges we have experienced in building such vertical scales and their implications for developing high-stakes assessments based on learning trajectories.