, the
right handed system rotated from 
ato
rb
r
 moves in the
direction of ˆn If either 
0 or
0
a
b
=
r=
r
r
r
, then θ is not defined and in this case, we define 
0
a
×b
r=
r
r Observations
1 a
×b
r
r
 is a vector