We say that a complex number is constructible if and are both constructible (we also say that the point is constructible). It is easy to show that is constructible iff the point can be constructed with a straight edge and compass in the cartesian plane starting with the points and . (Notice that our two definitions coincide when is a real number.)
It is possible to completely characterize the set of all constructible numbers:
A complex number is constructible iff it can be formed from the rational numbers in a finite number of steps using only the operations addition, subtraction, multiplication, division, and taking square roots.
For instance, this means one can construct segments of length: and , but one cannot construct a segment of length .
This condition can be rephrased in terms of field theory as follows:
A complex number is constructible iff there is a chain of field extensions such that each extension is quadratic (i.e. ).
This is equivalent because the field extension is quadratic iff for some with , so taking a square root in the above construction is equivalent to taking at most a quadratic extension of a field, while adding, subtracting, multiplying or dividing does not add anything to the field. (Does someone else want to phrase that better?)
Using this second characterization (and the tower law) we get the necessary (but not sufficient) condition that for some nonnegative integer , or equivalently that is algebraic and it's minimal polynomial has degree .