Encyclopaedia Britannica, 11th Edition, "Geodesy" to "Geometry" / Volume 11, Slice 6

by Various

Prop. 16. If four straight lines be proportionals, the rectangle contained by the extremes is equal to the rectangle contained by the means; and if the rectangle contained by the extremes be equal to the rectangle contained by the means, the four straight lines are proportionals.

In symbols, if a, b, c, d are the four lines, and

if

a : b = c : d,

then

ad = bc;

and conversely, if

ad = bc,

then

a : b = c : d,