The filigree is a difference in the thickness of the stamp paper that determines a legible design.

Not all stamps have filigree. There are often stamps of the same issue but with different watermarks. In recent years filigree has almost fallen into disuse.

Originally it was one of the elements that served to ensure anti-counterfeiting safety in stamps.

The largest number of varieties of watermarks are in Britain's stamps and its colonies which until the beginning of the 20th century used watermarks different from those established by the London government.

Hungary is the country that has most often changed filigree by passing from the simple letters "Kr" of the first emissions to the crown of Santo Stefano and then to the wavy crosses surmounted by the crown.

In the stamps of Finland, Norway, the Netherlands and Poland there is always mail horn. In Iceland, Denmark and Romania the royal crown is used while in Muslim countries the half moon is often present.

In Italy it was introduced in 1863 by Regie Poste, by the engineer Perazzi, who had designed the filigree "corona" to be used simultaneously with the printing of a security fund.

One of the most fascinating varieties of collectors is that of "phile letters".

It is already found in some stamps of the Kingdom period, normally with filigree "corona", and in the period of storage, both with filigree "corona" and "ruota wingata". In 1945 the Italian Republic introduced the filigree "alata wheel" and in 1955 the filigree "start" was introduced, which reached type IV, which was called Unit because it was used for the first time for the celebratory series of the Centenary of the Unity of Italy.

The supply of the paper was entrusted to the Cartiera Miliani of Fabriano who was so engaged in the manufacture of the coils using the system "in circle".

With this system the cellulose paste suitable for the manufacture of the paper, is passed on a drum consisting of a copper lattice that at drying happened, leaves the desired difference in level and forms the filigree.

The Republic of San Marino used the same watermarks in Italy until 1961, when it decided to adopt its own symbol: the watermark three feathers. The instrument used to examine the filigree is called filigreenoscope.