Page "Sinusoidal projection" Paragraph 7

The true distance between two points on a meridian can be measured on the map as the vertical distance between the parallels that intersect the meridian at those points.

With no distortion along the central meridian and the equator, distances along those lines are correct, as are the angles of intersection of other lines with those two lines.

Distortion is lowest throughout the region of the map close to those lines.