How it's made. The heel cup's inner face is the zero and every mark is laid off it in real millimetres, so the reading is the foot's length and nothing else. The stop rides in two slots, which keep it square - one slot lets it yaw and read a few millimetres off. It slides in from the open toe end and a keeper under the plate stops it lifting. 8 to 21 cm: newborn to about ten. Made flat in PLA, two colours, no supports.