The purpose of this article is to describe some apparatus which is easy to construct from materials that are reasonably cheap, and which has been found useful as a visual aid in many branches of elementary mathematics. It is thought that this apparatus will satisfy the need to which attention is drawn in § 4.8 of the Trigonometry Report, although it was being used before the report was published. After the apparatus has been described, some of its applications will be briefly indicated and a more detailed account of one application will follow.