This is an extension of String Graphs Part 1 but can be done independently. In this activity students stitch points on the lines y = x and y = -x. The family of straight lines form an envelope which can be modelled by finding successive points of intersection. The points of intersection can be modelled by a parabola. A range of extension questions are provided including generalising the parabola for stitching points on the lines y = mx and y = -mx.