Then we have d x − − d X d X = ε ε i j N i N j = N ⋅ ⋅ ε ε ⋅ ⋅ N where N i = d X i d X , is the unit vector in the direction of d X , and the left-hand-side expression is the normal strain e ( N ) in the direction of N . For the particular case of N in the X 1 direction, i.e., N = I 1 , we have e ( I 1 ) = I 1 ⋅ ⋅ ε ε ⋅ ⋅ I 1 = ε ε 11 .