Rule 3 guarantees correspondence between units of the L1 text and of the morphological gloss. It does not, however, insure that the vertical alignment works in a mechanical way. In certain situations, there may be good reasons not to insert morphological boundaries in T while still representing each morph by a separate gloss (Rule 23). Correspondence of boundary symbols in the L1 and the morphological gloss lines is therefore not generally an equivalence, but only an implication: boundary symbols appearing in T are matched by corresponding boundary symbols in the morphological gloss.

Rule 12. Apart from Rules 14 and 33, there is a boundary symbol of a certain type in the morphological gloss if there is a corresponding boundary symbol in the L1 text.
More strictly, there is a blank, hyphen, plus, equal sign, angled bracket and tilde in a morphological gloss if and only if there is an identical symbol in the L1 text corresponding to it.

The following sections are devoted to kinds of boundaries and their delimiters.