The angle of inclination required to view a girls knickers when she is wearing a skirt relative to a reference plane of the skirts hem.
A positive knicker angle requires your eye line to be above the hem of the skirt,
With a 0° knicker angle you can see knicker if your eye is level with the hem,
While a negative knicker angle requires you to be lower than the skirt and look up to observe knicker gusset.
Look at that tart at the bar, she's such a fuckin' slapper. I swear that microskirt she is wearing has positive knicker angle!