As common names for ‘expression line’ and ‘expression side’, on the one hand, and for ‘content line’ and ‘content side’, on the other, we have used respectively the designations ‘expression plane’ and ‘content plane’ […].
- Hjelmslev (1961), a pag.59

Just as the expression plane can, through a functional analysis, be resolved into components with mutual relations (as in the ancient discovery of alphabetic writing and in modern phonemic theories), so also the content plane must be resolved by such an analysis into components with mutual relations that are smaller than the minimal-sign-contents.
- Hjelmslev (1961), a pag.67