A relative-motion microworld has been designed to aid high-school students in understanding the concepts of relative motion and frames of reference. Relative motion and frames of reference are usually introduced in a high-school physics or mathematics course. Most students, and many teachers too, have difficulty understanding the concepts and applying them to solve problems. The traditional approach to relative motion uses vector algebra. However, vector terminology is complex. The relative-motion uses a new representation for thinking about motion, based on the intrinsic, local and procedural characteristics of turtle geometry. This representation provides the student with a simple framework for understanding motion and relative motion, that overcomes the problems associated with the traditional approach. The microworld allows students to set up their own motion problems, in order to view the motion from different reference frames. Four key primitives have been designed: these make an object, give an object a motion, select a reference frame, and move all the objects that have been set up. These four primitives are the building blocks that can be used to set up complex problems. Problems from a variety of motion situations can be explored using the relative-motion microworld. These include rectilinear motion, non-parallel rectilinear motion microworld. These include rectilinear motion, non-parallel rectilinear motion, motion under gravity, and rotational motion. A student text and teacher's reference manual accompany the microworld.