Machine Cancel - a cancellation applied by machine, as opposed to a hand cancel

Mailometer Perforations - The Mailometer Company of Detroit, Michigan, formed in 1906, used a vending machine invented by Joseph Schermack, then a production manager for the company. Many of the Mailometer type perforations are common.
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Manila Paper - a fiber-based and very coarse paper (originally from Manila hemp) often used for envelopes or newspaper wrappers

Manuscript Cancel - a cancellation of a stamp by pen or marker. Early stamps may show a manuscript cancel because the post office had no canceling device. Stamps with pen cancels are usually valued less than stamps with normal cancels. Today, stamps on envelopes or packages that were missed in the normal canceling process are pen cancelled by letter carriers.

Margin - the portion of unprinted paper that surrounds the design or perforated area (selvage) of a stamp or a pane of stamps

Margin Block - a block of stamps with the selvage still attached

Marginal Marking (Marginal Inscription) - any printing that appears in the margins. These might include the name of the printer, plate numbers, etc.

Merry Widow - (Scott E7) a nickname for the 1908 green Special Delivery stamp. The name came from the design of Mercury’s helmet on the stamp that resembled the hat worn by the widow from the opera of that name.
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Mint - a stamp in the same unused condition, including full gum, in which it came from the post office. "Mint" is casually, and incorrectly, used to include stamps with disturbed gum, with signs of previous hinging, and even with yellowed or toned stamps. It is often used as a synonym for "not cancelled" or "not used", although that is clearly not the literal meaning.

Mint Never Hinged (MNH) - MNH refers to the condition of having never had a hinge attached to the stamp. It implies that the gum is sound and intact and, in a sense, that the condition of the overall stamp is pristine. So much emphasis has been placed on the lack of gum disturbance from never having had a hinge attached to the stamp, that other factors such as bright fresh color is sometimes overlooked. There is no denying the fact that "MNH" can add value to an otherwise ordinary or even slightly sub-par stamp, but the first letter in MNH stands for mint, and we would argue that only stamps in pristine condition meet that standard.

Miscut - a stamp or pane that is cut so that parts of adjoining designs appear in the space normally occupied by the stamp design

Missionaries - a nickname for the earliest postage stamp issues of Hawaii, dating from the early 1850’s. The name derives from the fact that many of the stamps and covers that were saved were used by missionaries in Hawaii writing home to families in New England.

Mixed Franking - a properly used cover with the stamps of two or more nations affixed, or a cover with any unusual combination of stamps, such as an Official and a definitive or commemorative

Mobile Post Office (MPO) - a vehicular post office facility that sorts and distributes mail while in transit. Examples include while on train, on boat or ship, or on a streetcar or truck.

Molly Pitcher - the heroine of the Battle of Monmouth, N.J. Her name is overprinted on the two-cent stamp of 1928.
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Money Order - The first money orders were authorized by the U.S. on May 17, 1864 and went into effect November 1, 1864.

Mourning Cover - a cover with a black border, used to convey news of a death

Multiple - a group of two or more intact stamps, but less than a full sheet