Paper has long been used as a means to prevent forgery, particularly in the printing of banknotes and personal identity documents. However, other important documents such as checks, bonds, share certificates, lottery tickets, and even airline tickets are also commonly printed on security paper with features that are difficult or impossible to imitate.

One of the most effective security features of paper is the watermark. Watermarks are difficult to fake and easy to detect, making them a popular choice for preventing forgery. Simple watermarks can be created by impressing a dandy roll or pressing it into the paper while it is still wet. More complex half-tone marks can be made using vat formers, where the forming wire is shaped in a pattern corresponding to the watermark. The raised parts of the pattern correspond to light spots, while the depressions correspond to dark areas in the watermark.

Watermarks can be distributed as a pattern throughout the paper or incorporated into the design of the banknote. They are often positioned as a portrait in an unprinted margin or shield, adding an additional layer of security to the document.

Another traditional method of marking security paper is by mingling it with colored fibers. These mingling fibers are long, thin textile fibers that form a pattern of different colors embedded into the paper. This technique adds another level of complexity to the security paper, making it even more difficult to counterfeit.

As technology has advanced, more sophisticated security features have become necessary. For example, optically active fibers can be used in the mingling process, making the pattern visible only under ultraviolet light. Security threads with distinct patterns can also be embedded into the paper during the forming process. These threads can even be interlaced into the paper, partially embedded within the web and partially visible on the surface.

Banknote papers are specifically designed to resist extended use and wear. One important characteristic of banknote paper is its folding endurance, ensuring that it remains intact even after repeated folding. These papers are often made from cotton, linen, or hemp fibers and are sized with gelatin. The production of banknote paper typically takes place on small paper machines to ensure the highest level of quality and security.

In conclusion, the use of security paper with features difficult or impossible to imitate is a crucial aspect of preventing forgery. Watermarks, mingling fibers, optically active fibers, and embedded security threads are just a few of the techniques used to create secure documents. The strength and durability of banknote paper are also key factors in ensuring its longevity and resistance to wear. These security features and production processes are constantly evolving as technology advances, helping to protect important documents and currencies from counterfeiters.