Hairlight placement relative to subject

[TMR Design]

Typically, where is the hairlight placed relative to the subject?
Does it come from slightly behind? Directly from the side? or from slightly in front?

Or is that a creative decision based on preference? I am just starting out with the idea of hairlights and I would like a good starting point in terms of positioning?

[Wilt]

Most hairlight placement is behind. It could shine upward, downward, even straight toward the lens (obscurred by the head). A no-no is to ever let the light from this source fall onto any skin which is visible to the lens. The point is primarily to 'separate' the subject from the background. In black and white days, hues could not be used as visual cues because different hues could all end up the same shade of gray, so it was more important to use a hair light for separation than now. So tonal brightness differences helped with the separation. Now hairlights are often merely accents, and the separation issue is not present for some subjects and backgrounds

[chtgrubbs]

I usually put mine on the opposite side from the main light, slightly behind and 3-4 feet higher than the top of subjects head. I usually use a grid, snoot or barndoors to keep it from falling on the subjects skin. For blonds I use the same exposure as the main light, for brunettes add 1/2 stop.