"Dolf Overbeek was the head of the studio of the Vada printing firm, and around 1948 became the graphic adviser to De Arbeiderspers, a major Dutch publishing and printing house. Overbeek was an authoritative and demanding taskmaster, as well as the designer of prize-winning books and calendars. He was not fond of experiments and preferred conventional no-nonsense typography to fancy modernisms. Annoyed by bad typeface combinations, he analysed the compatibility of faces of different categories and designed the ‘Letterorgel’ (Letter Organ, after the musical instrument — a kind of scientific table) which prescribed exactly which combinations to use, and which to avoid."
From the book Dutch Type by Jan Middendorp:
"Dolf Overbeek was the head of the studio of the Vada printing firm, and around 1948 became the graphic adviser to De Arbeiderspers, a major Dutch publishing and printing house. Overbeek was an authoritative and demanding taskmaster, as well as the designer of prize-winning books and calendars. He was not fond of experiments and preferred conventional no-nonsense typography to fancy modernisms. Annoyed by bad typeface combinations, he analysed the compatibility of faces of different categories and designed the ‘Letterorgel’ (Letter Organ, after the musical instrument — a kind of scientific table) which prescribed exactly which combinations to use, and which to avoid."