Local coordinates mean a set of coordinates that travel with the particle, and have orientation determined by the path of the particle. Unit vectors are formed as shown in the image at right, both tangential and normal to the path. This coordinate system sometimes is referred to as "intrinsic" or "path coordinates" or "nt-coordinates", for "normal-tangential", referring to these unit vectors. These coordinates are a very special example of a more general concept of local coordinates from the theory of differential forms.