Intercept theorem

The Intercept Theorem, also known as the Basic Proportionality Theorem or Thales's theorem, states that if two parallel lines are intersected by two transversals, then the segments on the transversals are proportional. To be more precise, consider two parallel lines \( l_1 \) and \( l_2 \) cut by two transversals (lines) \( t_1 \) and \( t_2 \) that intersect them.