My first post on Nairaland, I hope it goes well

I am not Nigerian myself but a true friend of Nigeria in many ways.

A young child, with whom I'm very close, has a tribal mark on the right chin. It's one single black, thin line that runs vertically down the centre of the chin seen from the front. The line is only 1-2 centimetres long. The mark was made with a razor and dyed with charcoal.

Does anyone know the significance of this mark? I've been googleing for hours with poor success,