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

[![enter image description here][1]][1]

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


  [1]: https://i.sstatic.net/NSiZU.png