This article describes part of a study in which researchers designed lesson sequences based around using a string number line to help teachers support children's development of relative thinking and understanding of linear scale. In the first year of the study, eight teachers of Years 3-5 participated in four one-day professional development workshops (one per term), with implementation of the lesson sequences between each. The goal was to design and trial activities that could be easily implemented by teachers using simple resources and to determine their impact on students' ability to use proportional reasoning. The teachers gave their students a test prior to the first lesson and the same test was repeated in the week following the lesson sequence. In total, 204 students completed both pre- and post-tests. This approach proved effective for developing students' understanding of concepts related to scale and relative thinking as well as their mathematical language, which is essential in the development of conceptual understanding. It was also useful for teachers' own professional learning and as a means of assessing and monitoring their students' understanding and progress.