Instead of assigning raised numbers for morpheme classes, we assign a number for each boundary between morpheme classes: 'A¹ - An = ¹ N¹' instead of 'A¹ - An = N¹' ('darkeness' substitutable for 'light'). These numbers are considered part of the environment of the morpheme classes in question, on a par with the other morpheme classes which constitute the neighbors of the class in question, and the position of our class relative to this others, and the intonation or stress or other contour under which our morpheme class occurs […]. The boundary numbers are related to the raised numbers, but not identical with them. One of the advantages of the boundary numbers is that they indicate on which side the sequence is reaching a higher construction level (as in '³N ²', which shows that the noun phrase is closed on the left, since no part of the noun phrase can precede the article 'T', but not on the right, where an adjectival phrase such as 'from Washington' can still be added).
- Harris (1951), a pag.267, n.18