Editing Definable real number (section)

== Constructible numbers ==
{{main article|Constructible number}}
One way of specifying a real number uses geometric techniques. A real number <math>r</math> is a constructible number if there is a method to construct a line segment of length <math>r</math> using a compass and straightedge, beginning with a fixed line segment of length 1.

Each positive integer, and each positive rational number, is constructible. The positive square root of 2 is constructible.  However, the cube root of 2 is not constructible; this is related to the impossibility of [[doubling the cube]].