The Basic Proportionality Theorem, also known as Thales' Theorem, states the following:

Theorem: If a line is drawn parallel to one side of a triangle intersecting the other two sides, then it divides the two sides in the same ratio.

In simpler terms:

Consider a triangle ABC. If a line DE is drawn parallel to BC, where D is a point on AB and E is a point on AC, then the theorem states that:

AD/DB = AE/EC

Converse of the Basic Proportionality Theorem:

The converse of the theorem is also true. It states:

If a line divides any two sides of a triangle in the same ratio, then the line is parallel to the third side.