We can also learn from the construction formulae which morpheme classes are the heads and which are the closures of the sequences in which they appear: the closure is the class which always appears last; and the head is the class which can always substitute for the sequence, e.g. an 'N' morpheme for an 'N' –phrase sequence. The formulae can thus show which sequences are endocentric (e.g. 'A N = N') and which are exocentric (e.g. 'T A = N').
- Harris (1946), a pag.179