Found:

Labeled bracketing

**SYNTAX: **a way of representing the structure of an expression by writing square brackets ('[' and ']') to the left and right hand side of its component parts, i.e. words or **constituent**s. The brackets carry subscripts, so-called labels, which state the category of the unit in question.
** EXAMPLE:** the structure of the sentence *the boy may eat apples*
is written as in (i):

(i) [The bracket notation is equivalent to the representation by means of a_{IP}[_{NP}[_{Det}the][_{N}boy]][_{I'}[_{I}may][_{VP}[_{V}eat][_{NP}[_{N}apples]]]]]

(ii) [_{NP}[_{Det}the] [_{N}boy]] (iii) NP / \ Det N / \ the boy