General Postal Union - a union of nations established in 1874, later known as Universal Postal Union. The international union was formed to ensure the creation of a single postal territory for the reciprocal exchange of correspondence among countries.
Grill - a pattern of small, square dots in an overall square or diamond-shape that is applied to stamps to prevent their reuse after cancellation. The embossing breaks the paper fibers of the stamp, allowing the canceling (obliterating) ink to penetrate effectively and preventing this ink from being removed by would-be defrauders.
Guerrilla stamp - a stamp issued by guerrilla forces to frank their correspondence. Use of such stamps was common in Taiwan (1895), Philippines (1898), South Africa (1899-1902), Ireland (1922-23), China (1929- ), and South Vietnam (1963-76).
Guide lines - horizontal and vertical lines that extend wholly or partially across sheets to facilitate perforating and cutting into panes. U.S. stamps are normally sold in panes of one hundred subjects. Blocks of stamps bearing such lines are designated as a 'guide-line pair' or 'block', and blocks showing intersecting lines are termed 'center-line blocks'.