When the linear and vector function expressing a strain is self-conjugate the strain is pure. When it is not self-conjugate, it may be broken up into pure and rotational parts in various ways (analogous to the separation of a quaternion into the sum of a scalar and a vector part, or into the product of a tensor and a versor part), of which two are particularly noticeable. Denoting by a bar a self-conjugate function, we have thus either where є is a vector, and q a quaternion (which may obviously be regarded as a mere versor).