
Dividers.

Compasses are made of wood or metal, in all sizes, with or without a sector.



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According to the Poncelet–Steiner theorem, every straightedge-and-compass construction can be carried out with a straightedge alone, provided that a fixed circle and its center are given. But how is this done in practice? Though the question may seem playful or even pointless, it has in fact attracted the attention of several mathematicians.

How can you determine your position when sailing offshore or along the coast? With land in sight, one or two landmarks are enough. On the open sea, sailors’ ingenuity led to the invention of sophisticated instruments such as the sextant and the Cras plotter. A little geometry is essential to use them.

How long have schoolchildren been drawing geometric figures with a straightedge and compass? More importantly, why? Even elementary geometric constructions must be introduced progressively, and this requires knowledge to be organized systematically.

Since antiquity, navigators have sought to find their position at sea, first developing instruments and then maps before the practice was standardized in the 19th century. This practical and strategic use of mathematics endured for centuries before other tools superseded it.
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