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Semi-postal stamp
Costs more than the price of postage. The extra money is donated to a charity or special fund.

Series
All the variations of design and value of a particular issue.

Set
Unit of stamps often with a common theme, usually commemorative, issued together.

Set-tenant
Stamps of different designs, colours, or values printed side by side in the same pane.

Souvenir sheet
Small sheet of stamps with a commemorative inscription.