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Zanzibar, Tanzania, comprises a pair of islands off the east coast of Africa called Zanzibar ('Unguja') and Pemba. They are, together with Mafia Island, sometimes referred to as the Spice Islands.
In 1964, Tanganyika united with the island of Zanzibar, forming the United Republic of Tanganyika and Zanzibar, later renamed to the United Republic of Tanzania. Although Zanzibar is part of Tanzania, it elects its own president who is head of government for matters internal to the island.
See: Tanzania stamps
Inside #228: Stamps on Envelopes