Surcharges
The term "surcharge" in philately describes any type of overprint that alters the price of a stamp. Surcharges raise or lower the face value of existing stamps when prices have changed too quickly to produce an appropriate new issue, or simply to use up surplus stocks.
Any overprint which restates a stamp's face value in a new currency is also described as a surcharge. Some postal systems have resorted to surcharge overprints when converting to a new national monetary system, such as when British Commonwealth countries converted to decimal currency in the late 1960s.
Stamps have occasionally been overprinted multiple times. A famous example of repeated surcharging happened during the German hyperinflation of 1921–1923. Prices rose so fast and dramatically that postage stamps which cost five or ten pfennigs in 1920 were overprinted for sale in the values of thousands, millions, and eventually billions of marks.
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Victoria, 1873: Penny stamp overprinted to new value of halfpenny.
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Ceylon, 1888: UK stamp surcharged by a "double inverted" overprint.
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Russia, 1919: Siberian 3-kopeck overprinted to new value of 50 kopecks.
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Germany, 1923: Five thousand mark value overprinted to two million.
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