I'm puzzled by the term "Duotone". In Photoshop there's a feature which allows the user to alter the darker tones in a grey toned image away from pure grey to another tint/colour. Looking at David Bailey's 1992 book "If we shadows" it seems well printed but the book states that the plates are "Duotones". I'm afraid that I don't see any hint of colour in the images - they're all shades of (dark) neutral grey from black to clear paper, as far as I can tell. Could it have been used at the time to suggest a better (darker?) quality of printing? In his later book, "Democracy", the printing is generally lighter, and I think, somewhat better - there's no mention of "Duotone". Or does the term have quite different meanings in the print domain as opposed to its use in Photoshop?