Here is an extract from the very first measures of Le Marteau sans maître from Pierre Boulez. My problem is, I can’t figure out what these strange vertical lines mean. (And I’m not talking about the barlines ;) )
Does anyone have an idea?
That is likely a musical separator. Typically, they are used as a sort of punctuation, to illustrate phrasing or motivic structure and sequences. In some contexts they might be akin to phrase markings. It is doubtful the composer wrote them in himself (at least it is for anything before contemporary music) and could be an editor's addition.