One of the oldest and most reliable security elements present in paper. It is formed during the paper manufacture, still in the wettest stage where the sheet is formed, and thus, becomes an intrinsic part of the substrate. It can be seen when the paper is placed against a light source due to the variation in translucency obtained by the different concentration of cellulose fibers, which in turn forms the watermark image on the sheet.

In general, watermarks can be:

• Single Tone: present when the image formed is only light or only dark.
• Duotone: present when the image is formed by elements of well-defined light tones and dark tones.
• Multi-tone: present when the image formed has several halftones.