What is "cancelled-to-order"

A cancelled to order (also called and abbreviated CTO) postage stamp is a stamp the issuing postal service has cancelled (marked as used), but has not traveled through the post, but instead get handed back to a stamp collector or dealer. They can come from withdrawn stocks of stamps cancelled in sheets and sold as remainders or from new sheets for sale at reduced rates to the stamp trade. Postal services of various countries do this in response to collector demand, or to preclude stamps issued for the collector market being used on mail. Some of the history of CTOs is from stamps being given to collectors on an approval basis, in person or through mailings; the first CTOs began in the late 19th century.