In what case is it said that the segments AB and CD are proportional to the segments A1 B1 and C1 D1?

They say that the segments AB and CD are proportional to the segments A1 B1 and C1 D1 if ab / a1b1 = cd / c1d1. The segments are called proportional – the segments for the lengths of which the proportion is fulfilled. The ratio of segments AB and CD is the ratio of their lengths, that is. Segments AB and CD are proportional to segments A1B1 and C1D1, if: for example, segments AB 2 cm long and CD 1 cm long are proportional to segments A1B1 3 cm long and C1D1 1.5 cm long. Thales’ theorem uses the concept of proportional line segments.