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Male pattern baldness and beards need testosterone so there's no need to ask.

Using a straight for your armpits as well?

Sure you can, but would it be practical? I never tried but I guess pulling out an armpit hair is more painful, and you have to undress for it first unless you start collecting them every day before...

That's the problem I have with most hairs when trying to find one in a brush.
Beard hairs are even thicker than scalp hair, so I always look for a thick one for the HHT.

In my experience bristle of an old brush is more friable than human hair. In other words: all my razors that pass the HHHT (i.e. hanging human hair test) pass the HBBT (i.e. hanging brush bristle...

Get them from a hairdresser's or your S.O.'s hairbrush!

Good luck!