A security thread is a security feature of many bank notes to protect against counterfeiting, consisting of a thin ribbon that is threaded through the note's paper

Paper

Paper is a thin material mainly used for writing upon, printing upon, drawing or for packaging. It is produced by pressing together moist fibers, typically cellulose pulp derived from wood, rags or grasses, and drying them into flexible sheets....

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Usually, the ribbon runs vertically, and is "woven" into the paper, so that it at some places emerges on the front side and at the remaining places at the rear side of the paper. Usually, it is made of metal

Metal

A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...

 foil

Foil (chemistry)

A foil is a very thin sheet of metal, usually made by hammering or rolling a piece of metal. Foils are most easily made with malleable metals, such as aluminium, copper, tin, and gold. Foils usually bend under their own weight and can be torn easily. The more malleable a metal, the thinner foil can...

, but sometimes of plastic

Plastic

A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs...

. Often, it has some text or numbers (e.g., the denomination) engraved

Engraving

Engraving is the practice of incising a design on to a hard, usually flat surface, by cutting grooves into it. The result may be a decorated object in itself, as when silver, gold, steel, or glass are engraved, or may provide an intaglio printing plate, of copper or another metal, for printing...

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Threads are embedded within the paper fiber and can be completely invisible or have a star burst effect, where the thread appears to weave in and out of the paper when viewed from one side. However when held up to the light the thread will always appear as a solid line. Features can be built into the thread material e.g., microprinting

Microprinting

Microprinting is one of many anti-counterfeiting techniques used most often on currency and bank checks, as well as various other items of value. Microprinting involves printing very small text, usually too small to read with the naked eye, onto the note or item. Microprint is frequently hidden in...

 on a transparent plastic thread or adding materials so they fluoresce under ultraviolet light.
The thread is a difficult feature to counterfeit but some counterfeiters have been known to print a thin grey line or a thin line of varnish in the area of the thread.

Security threads can also be used as an anti-counterfeiting device in passport

Passport

A passport is a document, issued by a national government, which certifies, for the purpose of international travel, the identity and nationality of its holder. The elements of identity are name, date of birth, sex, and place of birth....

s. They are generally made of plastic and contain microprinting.