A number which can be represented by a finite number of additions, subtractions, multiplications, divisions, and finite square root extractions of integers. Such numbers correspond to line segments which can be constructed using only straightedge and compass.
All rational numbers are constructible, and all constructible numbers are algebraic numbers (Courant and Robbins 1996, p. 133). If a cubic equation with rational coefficients has no rational root, then none of its roots is constructible (Courant and Robbins 1996, p. 136).
In particular, let 
be the field of rationals. Now construct an
extension field 
of constructible numbers by the adjunction of 
, where 
is in 
, but 
is not, consisting of all numbers of
the form 
,
where 
.
Next, construct an extension field 
of 
by the adjunction of 
, defined as the numbers 
, where 
, and 
is a number in 
for which 
does not lie in 
. Continue the process 
times. Then constructible numbers are precisely those which
can be reached by such a sequence of extension fields 
, where 
is a measure of the "complexity" of the construction
(Courant and Robbins 1996).
