Ruler or Straight-edge
Rulers come in all sorts of shapes and sizes. Wooden, plastic or metallic, there is really no big difference, in fact any straight edge will do. However, we suggest to choose a ruler that is at least 30cm long.
With the ruler we can draw lines, and it can also be used to help fold paper when building 3D models. The metallic ruler is a better option, should you use a knife to cut out geometric templates.