Poncelet–Steiner theorem

In Euclidean geometry, the Poncelet–Steiner theorem concerning compass and straightedge constructions states that whatever can be constructed by straightedge and compass together can be constructed by straightedge alone, provided that a single circle and its centre are given. This result can not be weakened; if the centre of the circle is not given, it cannot be constructed by a straightedge alone. Also, the entire circle is not required. In 1904, Francesco Severi proved that any small arc together with the centre will suffice.

The result was conjectured by Jean Victor Poncelet in 1822, and proven by Jakob Steiner in 1833.

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