An indication on a cover that postage is prepaid, partially prepaid or that the letter is to be carried free of postage. Franks may be written, hand-stamped, imprinted or affixed. Free franking is usually limited to soldiers' mail or selected government correspondence. Postage stamp and postage meter stamps are modern methods of franking a letter.

An abnormal, usually nonre-petitive occurrence in the production of stamps that results in a variation from the normal stamp, but falls short of producing an error. Most paper folds, overinking and perforation shifts are freaks. Those abnormalities occurring repetitively are called varieties and may result in major errors.

The front of a cover with most or all of the back and side panels torn away or removed. Fronts, while desirable if they bear unusual or uncommon postal markings, are less desirable than an intact cover.

Printing inks used in stamp production that easily fade or break up in water or chemicals. To counter attempts at forgery or the removal of cancellations, many governments have used fugitive inks to print stamps.