What is the most commonly used security feature in paper currency worldwide?

If you hold a banknote up to the light, you will likely see a faint, ghost-like image embedded within the paper itself—often a portrait, a number, or a national symbol. This image is not printed on the surface but is created during the papermaking process by varying the paper's thickness and density. Thinner areas allow more light to pass through, creating the lighter parts of the image, while thicker areas appear darker. Because this feature is integral to the paper, it is extremely difficult to convincingly replicate with a standard printer.

This simple yet elegant security technique is the most common and enduring anti-counterfeiting measure used in currency around the world. Its origins are surprisingly old, dating back to 13th-century Italy, where papermakers first used these marks to identify their work and prevent imitation. Banks later adopted the method, with the Bank of England incorporating them as early as 1697. While modern banknotes are now layered with high-tech defenses like holographic strips, color-shifting inks, and microprinting, this classic feature remains a foundational and universally recognized first line of defense against forgery.