"Aux trois crayons" is a technique that uses a limited pallet of 3 out of 4 colours (sanguine, sepia, black and white), usually on a tinted paper.
The resulting pictures can look more "alive" than a charcoal or pencil drawing.
The medium is fairly granular (high clay content), and when used in combination with a "toothed" paper, it doesn't provide quite the same smoothness or level of detail as charcoal and pencil, however, the warmth of the colours compensate for this.
I frequently add a fourth colour to produce my variation on this drawing technique, which enjoyed popularity in the 15th and 16th century. This medium is ideal for (but not limited to) working from low quality photographs.
Charcoal and pencil can similarly be combined with chalk for a more dynamic monochromatic effect than charcoal/pencil alone.
