For starters, we shall discuss the non balding Asian/ Caucasian male face (at 30 to 35 years age).
There are 3 points, I would like you to concentrate on
A - the centre lowermost point of the hairline
B - also called the temple angle (the point where the front and sides meet)
C - the temple point (self explanatory term - the pointed tip of hair seen in the profile view).
The middle half of the hairline (in most adult males), runs parallel to the middle half of the eyebrow.
Look at any face/picture. Draw an imaginary line along the middle half of the hairline outwards (line 1).
Now draw another imaginary line over the mid half of the eyebrow and carry it outwards (line 2).
Non balding adult male - Line 1 and line 2 run parallel to each other.
When line 1 and line 2 diverge, that becomes a mature hairline.
Line 1 and 2 converge in feminine or teenage hairlines.
Thus, when you LOOK, you are now looking at the parallel configuration of the mid half of the hairline and the mid half of the eyebrow.
Now lets look at a few real life examples
Please note that the negroid hairline is an exception. There, often line 1 and 2 converge.