Main plate: Base plate on which all the other parts of a watch movement are mounted.

Mainspring: The driving spring of a watch or clock, contained in the barrel.

Manual: A hand-wound mechanical watch.

Manufacture: French term for a watch factory which itself produces the components needed for the manufacture of its products (watches, alarm and desk clocks, etc).

Marine chronometer: Highly accurate mechanical or electronic timekeeper enclosed in a box (hence the term box chronometer), used for determining the longitude on board ship.Marine chronometers with mechanical movements are mounted on gimbals so that they remain in the horizontal position is necessary for their precision.

Mean time: A scale of time-measurement in which the units – second, minute, hour etc. – are of the same length throughout the year. It is easily forgotten nowadays that this is not true of the sun's time, by which the length of a 24-hour period shows a variation of up to 30 minutes 46 seconds if measured by a constant-rate timekeeper. In the 18th century, ‘equation tables’ were published showing the difference between solar time and mean time throughout the year, so that clocks and sundials could be correlated.

Mechanical movement: A movement based on a mainspring which is wound by hand; when wound, it slowly unwinds the spring in an even motion.

Middle (of watch-case) Movement: Assembly consisting of the principal elements and mechanisms of a watch or clock: the winding and setting mechanism, the mainspring, the train, the escapement, the regulating elements. "Anatomically", the movement consists of the "ébauche", the regulating elements and the other components.

Military or 24-hour time: Time is measured in 24-hour segments. To convert 12-hour time into 24-hour time, simply add 12 to any p.m. time. To convert 24-hour time into 12-hour time, subtract 12 from any time from 13 to 24.

Mineral glass: Watch glass that has been tempered to increase its scratch resistance. It is considered more shatter resistant than sapphire, though not as scratch resistant.

Mint in box (MIB): Items may have been purchased but never used; in pristine condition.

Minute repeater: A complication on a watch that can strike the time in hours, quarters, or seconds. Especially coveted (and very expensive) in mechanical movements.

Moon phase: An indicator that keeps track of the phases of the moon. A regular rotation of the moon is once around the earth every 29 days, 12 hours, and 44 minutes. once set, the moon phase indicator accurately displays the phase of the moon.

Mother-of-Pearl: Iridescent, milky interior shell of the fresh water mollusk that is sliced thin and used on watch dials. While most have a milky white luster, mother-of-pearl also comes in other colors such as silvery gray, gray blue, pink, and salmon.

Movement: The inner workings or assembly that make up the main timekeeping mechanism. movements are either quartz or mechanical. This is the engine of the watch.

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Navette: Elongated gemstone cut, synonymous with the marquise cut.

New old stock (NOS) - items considered older but have never actually passed through the hands of the general public. They are as they were shipped from the factory.

Nickel silver: Copper, zinc and nickel alloy, oxidizes less readily than brass.

Nivaflex: the trade name for the spring material made by Nivarox FAR, a company of the Swatch group. These materials self compensate for the effects of temperature (e.g. expansion and contraction), are extremely strong and corrosion resistant.

Nivarox: Swiss company (full business name Nivarox - FAR SA) formed by a merger in 1984 between Nivarox SA and Fabriques d'Assortiments Réunis (FAR). It is currently owned by the Swatch Group. Nivarox is also the trade name of the metallic alloy from which its products are fabricated. Nivarox is most famous for producing hairsprings which are attached to the balance wheel inside a mechanical watch movement, as well as mainsprings which provide the motive power for the watch.

Numeral: Character representing a number used to indicate the units of time on the dial. The Arabic, Roman or Breguet style numerals may be flat (painted) or raised (which may be sculpted or applied, also referred to as appliques). They are different from symbolic characters such as hour markers.