A number of ancient problems in geometry involve the construction of lengths or angles using only an idealized ruler and compass.
The most famous ruler-and-compass problems have been proven impossible, in several cases by the results of Galois theory. In spite of these impossibility proofs, some mathematical novices persist in trying to solve these problems. Many of them fail to understand that many of these problems are trivially solvable provided that other geometric transformations are allowed: for example, squaring the circle is possible using geometric constructions, but not possible using ruler and compass alone.
Mathematician Underwood Dudley has made a sideline of collecting false ruler-and-compass proofs, as well as other work by mathematical cranks, and has collected them into several books.
The compass can be opened arbitrarily wide, but (unlike most real compasses) it also has no markings it. It can only be opened to widths you have already constructed.
Each construction must be exact. Eyeballing it and getting close does not count as a solution.
Stated this way, ruler and compass constructions are a parlor game, rather than a serious practical problem. Figuring out how to do any particular construction is an interesting puzzle, but the persistent interest in the problem derived from what you can’t do this way.
The three classical unsolved construction problems were:
Squaring the circle: Drawing a square the same area as a given circle.
Doubling the cube: Drawing a cube with twice the volume as a given cube.
Trisecting the angle: Dividing a given angle into three smaller angles all of the same size.
For 2000 years people tried to find constructions within the limits set above, and failed. The reason? Because all three are impossible.
You can identify a point (x,y) in the Euclidean plane with the complex number x + yi.
In ruler and compass construction, one starts with a line segment of length one. If one can construct a given point on the complex plane, then one says that the point is constructible. By standard constructions of Euclidean geometry one can construct the complex numbers in the form x.+ yi with x and yrational numbers. More generally, using the same constructions, one can, given complex numbers a and b, construct a + b, a - b, a * b, and a / b. This shows that the constructible points form a field, which one treats as a subfield of the complex numbers. Moreover, one can show that the given a constructible length one can construct its complex conjugate and square root.
The only way to construct points is as the intersection of two lines, of a line and a circle, or of two circles. Using the equations for lines and circles, one can show that the points at which they intersect lie in a quadratic extension of the smallest field F containing two points on the line, the center of the circle, and the radius of the circle. That is, they are of the form x + y √ k, where x, y, and k are in F.
Since the field of constructible points is closed under square roots, it contains all points that can be obtained by a finite sequence of quadratic extensions of the field of complex numbers with rational coefficients. By the above paragraph, one can show that any constructible point can be obtained by such a sequence of extensions. As a corollary of this, one finds that the degree of the minimal polynomial for a constructible point (and therefore of any constructible length) has degree a power of 2. In particular, any constructible point (or length) is an algebraic number.
Squaring the circle has been proved impossible, as it involves generating a transcendental ratio, namely 1:√π.
Only algebraic ratios can be constructed with ruler and compass alone. The phrase "squaring the circle" is often used to mean "doing the impossible" for this reason.
Without the constraint of requiring solution by ruler and compass alone, the problem is easily soluble by a wide variety of geometric and algebraic means, and has been solved many times in antiquity.
Using the trigonometric identity cos(3α) = 4cos³(α) - 3cos(α), one sees that, letting cos 20° = y, that 8y³ - 6y - 1 = 0, so, substituting x = 2y, x³ - 3x - 1 = 0. The minimal polynomial for x is a factor of this, but if it were not irreducible, then it would have a rational root which, by the rational root theorem, must be 1 or -1, which are clearly not roots. Therefore the degree for the minimal polynomial for cos 20° is of degree three, so cos 20° is not constructible and 60° cannot be trisected.
Some regular polygons (e.g. a pentagon) are easy to construct with ruler and compass; others are not. This led to the question being posed: is it possible to construct all regular polygons with ruler and compass?
Carl Friedrich Gauss in 1796 showed that a regular n-sided polygon can be constructed with ruler and compass if the odd prime factors of n are distinct Fermat primes. Gauss conjectured that this condition was also necessary, but he offered no proof of this fact, which was proved by Pierre Wantzel in (1836). See constructible polygon.
It is possible, as shown by Georg Mohr, to construct anything with just a compass that can be constructed with ruler and compass.
It is impossible to take a square root with just a ruler, so some things cannot be constructed with a ruler that can be constructed with a compass; but given a circle and its center, they can be constructed.