be known as dummy or training stamps Variable value stamps - dispensed by machines that print the cost of the postage at the time the stamp is dispensed War tax stamp – a variation on the postal tax stamp to defray the cost of war Water-activated stamp – For many years, water-activated stamps were the only type available, so this term entered into use with the advent of self-adhesive stamps. The adhesive or gum on a water-activated stamp must be moistened (usually by licking, thus the stamps are also known as "lick and stick"). Apart from these, there are also revenue stamps (used to collect taxes or fees on items like documents, tobacco, alcoholic drinks, hunting licenses, and medicines) and telegraph stamps (for sending telegrams), which fall in a separate category from postage stamps. First day covers Postage stamps are first issued on a specific date, often referred to as the First day of issue. A first day cover usually consists of an envelope, a postage stamp and a postmark with the date of the stamp's first day of issue thereon. Starting in the mid-20th century some countries began assigning the first day of issue to a place associated with the subject of the stamp design, such as a specific town or city. There are two basic types of First Day Covers (FDCs) noted by collectors. The first and often most desirable type among advanced collectors is a cover sent through the mail in the course of everyday usage, without the intention of the envelope and stamp ever being retrieved and collected. The second type of FDC is often referred to as "Philatelic", that is, an envelope and stamp sent by someone with the intention of retrieving and collecting the mailed item at a later time and place. The envelope used for this type of FDC often bears a printed design or cachet of its own in correspondence with the stamp's subject and is usually printed well in advance of the first day of issue date. The latter type of FDC is usually far more common; it is usually inexpensive and relatively easy to acquire. Covers which were sent without any secondary purpose are considered non-philatelic and often are much more challenging to find and collect. Souvenir or miniature sheets Postage stamps are sometimes issued in souvenir sheets or miniature sheets containing one or a small number of stamps. Souvenir