Where is the wing sweep angle measured from?

If you look at a 747 planform wing you see something like this:

In several aerodynamics and aircraft design text books they use the wing sweep angle as an important parameter to measure the wing efficiency at transonic velocities, but they don't specify from where they measure that angle. Even I've seen wings with double leading edge angles.

The sweep angle $ϕ$ depends on the % line on which it is measured. Normally the sweep angle of the leading edge $ϕ_{LE}$, trailing edge $ϕ_{TE}$, 25% line $ϕ_{25}$ (quarter chord sweep) and 50% line $ϕ_{50}$ are stated.
So normally in design 4 angles will be stated. Otherwise, e.g., when checking Wikipedia, the references used should typically be clear. Below shows two $ϕ_{25}$ angles, for example: