The intercept theorem, also known as Thales's theorem or basic proportionality theorem, is an important theorem in elementary geometry about the ratios of various line segments that are created if two intersecting lines are intercepted by a pair of parallels. It is equivalent to the theorem about ratios in similar triangles. Traditionally it is attributed to Greek mathematician Thales.^{[1]}
Suppose S is the intersection point of two lines and A, B are the intersections of the first line with the two parallels, such that B is further away from S than A, and similarly C, D are the intersections of the second line with the two parallels such that D is further away from S than C.
The first intercept theorem shows the ratios of the sections from the lines, the second the ratios of the sections from the lines as well as the sections from the parallels, finally the third shows the ratios of the sections from the parallels.
The intercept theorem is closely related to similarity. It is equivalent to the concept of similar triangles, i.e. it can be used to prove the properties of similar triangles and similar triangles can be used to prove the intercept theorem. By matching identical angles you can always place two similar triangles in one another so that you get the configuration in which the intercept theorem applies; and conversely the intercept theorem configuration always contains two similar triangles.
In a normed vector space, the axioms concerning the scalar multiplication (in particular and ) ensure that the intercept theorem holds. One has
There are three famous problems in elementary geometry which were posed by the Greeks in terms of compass and straightedge constructions:^{[2]}^{[3]}
It took more than 2000 years until all three of them were finally shown to be impossible with the given tools in the 19th century, using algebraic methods that had become available during that period of time. In order to reformulate them in algebraic terms using field extensions, one needs to match field operations with compass and straightedge constructions (see constructible number). In particular it is important to assure that for two given line segments, a new line segment can be constructed such that its length equals the product of lengths of the other two. Similarly one needs to be able to construct, for a line segment of length , a new line segment of length . The intercept theorem can be used to show that in both cases such a construction is possible.
To divide an arbitrary line segment in a ratio, draw an arbitrary angle in A with as one leg. On the other leg construct equidistant points, then draw the line through the last point and B and parallel line through the mth point. This parallel line divides in the desired ratio. The graphic to the right shows the partition of a line segment in a ratio.^{[4]} 
According to some historical sources the Greek mathematician Thales applied the intercept theorem to determine the height of the Cheops' pyramid.^{[1]} The following description illustrates the use of the intercept theorem to compute the height of the pyramid. It does not however recount Thales' original work, which was lost.
Thales measured the length of the pyramid's base and the height of his pole. Then at the same time of the day he measured the length of the pyramid's shadow and the length of the pole's shadow. This yielded the following data:
From this he computed
Knowing A,B and C he was now able to apply the intercept theorem to compute
The intercept theorem can be used to determine a distance that cannot be measured directly, such as the width of a river or a lake, the height of tall buildings or similar. The graphic to the right illustrates measuring the width of a river. The segments ,, are measured and used to compute the wanted distance . 
The intercept theorem can be used to prove that a certain construction yields parallel line (segment)s.
If the midpoints of two triangle sides are connected then the resulting line segment is parallel to the third triangle side (Midpoint theorem of triangles). 
If the midpoints of the two nonparallel sides of a trapezoid are connected, then the resulting line segment is parallel to the other two sides of the trapezoid. 
An elementary proof of the theorem uses triangles of equal area to derive the basic statements about the ratios (claim 1). The other claims then follow by applying the first claim and contradiction.^{[5]}
Since , the altitudes of and are of equal length. As those triangles share the same baseline, their areas are identical. So we have and therefore as well. This yields and Plugging in the formula for triangle areas () transforms that into and Canceling the common factors results in: (a) and (b) Now use (b) to replace and in (a): Using (b) again this simplifies to: (c) 
Draw an additional parallel to through A. This parallel intersects in G. Then one has and due to claim 1 and therefore

Assume and are not parallel. Then the parallel line to through intersects in . Since is true, we have 
Claim 4 can be shown by applying the intercept theorem for two lines.