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When pressure-sensitive adhesive made its first appearance on postage stamps in the 1960s, most stamp collectors regarded it as a gimmick. Also called self-adhesive, or self-stick, or peel-and-stick stamps, they represented a radical departure from traditional lick-and-stick stamps that had employed water-activated adhesive since 1840.
The United States was not the first country to put pressure-sensitive gum on stamps. For the first three countries to issue self-stick stamps—Sierra Leone in 1964, Tonga in 1969, and Bhutan in 1969—an alternative to water-activated gum was practical. All three countries have humid climates, in which old style gum has a tendency to get messy when days and nights are damp for extended periods of time.
http://www.askphil.org/ap_salm05.htmSteve


