issued for use by the government or a government agency Occupation stamp – a stamp for use by an occupying army or by the occupying army or authorities for use by civilians Non-denominated postage – postage stamp that remains valid even after the price has risen. It is also known as a permanent or "forever" stamp. Overprint – a regularly issued stamp, such as a commemorative or a definitive issue, that has been changed after issuance by "printing over" some part of the stamp Denominations can be changed in this manner. Perforated stamps – While this term usually refers to perforations around a stamp to divide a sheet into individual stamps, it can also be used for stamps perforated across the middle with letters or a pattern or monogram. They are known as "perfins". The modified stamps are usually purchased by corporations to guard against theft by employees. Personalised stamps – allow the user to add their own image Pneumatic post stamps – for mail sent using pressurized air tubes, only produced in Italy Postage and revenue stamps – stamps which were equally valid for postal and fiscal use Postage currency postage stamps used as currency rather than as postage Postage due – a stamp showing that the full postage has not been paid, and indicating the amount owed The United States Post Office Department has issued "parcel post postage due" stamps Postal tax – a stamp indicating that a tax above the postage rate required for sending letters has been paid This is often mandatory on mail issued on a particular day or for a few days. Poster stamp Self-adhesive stamp – not requiring moisture to stick, self-sticking Semi-postal / charity stamp – a stamp with an additional charge for charity The use of semi-postal stamps is at the option of the purchaser. Countries including Belgium and Switzerland often use charitable fund-raising design stamps which are desirable for collectors. Souvenir sheet – a commemorative issue in large format valid for postage often containing a perforated or imperforate stamp as part of its design See also minisheet. Specimen stamp – sent to postmasters and postal administrations so that they are able to identify valid stamps and to avoid forgeries Test stamp – a label not valid for postage, used by postal authorities to test sorting and cancelling machines or machines that can detect a stamp on an envelope May also