A dark graphite line that has been meticulously plotted.

The original system used graphite lines on the back of the stamp.

Coat the cutting area with the chalk, then draw a graphite line on the white surface.

A cupboard is actually a painting with graphite lines to indicate doors; it's like a Minimalist triptych altarpiece.

The Arundel series of paintings, begun in 1973, features barely visible graphite lines and accumulations of white paint on white surfaces.

Katherine Jackson's pieces are the simplest, with graphite lines moving across white surfaces.

In the United Kingdom, graphite lines were used exclusively on five stamps of the Wilding series from December 1957.

Several other errors of missplaced graphite lines are also known and not all are included in the Stanley Gibbons catalogue.

The machine used scanners to detect the graphite lines and to face the letters so that the stamp was always in the top right corner.

The machine could also sort the letters according to the number of graphite lines on the stamps so that fully paid and printed matter stamps were separated.