Duotone
A print made from two inks, typically black plus one colour, giving a monochrome image more tonal depth than black alone.
Measured in
count
In depth
A single black ink on press can render only a limited number of distinguishable tones, so a second ink is added to extend the range and warm or cool the image. Each ink is given its own curve, letting one carry the shadows and the other the midtones — which is what gives fine art books their tonal richness.
Examples in practice
A photobook prints its monochrome plates as duotones, using black for shadows and a warm grey for midtones.