Unit Normal to a Plane Curve

The unit outwards normal to a curve in the  
  plane can be derined in terms of the unit tangent vector  
  to the curve taken counterclockwise and the unit vector  
  along the  
  axis by considering a right angled cpprdinate system on the curve.

For the  
\[\mathbf{n} \mathbf{T} \mathbf{k}\]
  system above  
\[\mathbf{k} \times \mathbf{T} = \mathbf{n}\]