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Calendar watches - have subdials or pointers indicating the month, date and sometimes day of the week.
    Simple Calendar -     shows the Date of the month only
    Day-Date Calendar - shows the Date and the Day of the week
    Complete Calendar - shows the Date, Day of the week and the Month

Calibre - the size or style of a watch movement.  Cal. = short form.

Cannon Pinion - The tube fitting on the center wheel pivot that carries the minutes hand.

Carat - 1 carat = 1/24 of fine gold. 18 carat gold contains 18/24 fine gold or 75% gold content.

Case - the container that protects the watch movement. It also gives the watch an attractive appearance. Cases come in many shapes, round, square, oval, rectangular. A case is called "shaped" when it is not round. Cases can be made of different metals including stainless steel, gold, titanium, silver and platinum.

Caseback - the underside of a watch that lies against the skin. Some casebacks are made of crystal allowing you to view the watch movement.

Chamfering - bevelling the edges of bridges, screws.

Chevreau - in a watch band means that it is made from kid leather.

Chronograph - watches with a built-in stopwatch function. There are 2 independent time systems. One indicates the time of day and the other measures intervals of time. Subdials are used to keep track of seconds and of elapsed minutes and hours. Counters can be started and stopped as desired.

Chronometer - An instrument for measuring time very accurately. For a Swiss watch to be called a chronometer it must meet very high standards set by the C.O.S.C. (Contrôle Officiel Suisse des Chronomètres). The requirements are 15 days and nights at 5 different positions and temperature changes.
The Organization of Swiss Watch Manufacturers definition as of 1951: "A Chronometer is a precision watch which is regulated in various positions and at different temperatures and has received a certificate to that effect".

Co-axial - the hour and minute hands move around the same axis.

Coin Watch - a coin is used as the case.

Complication - Functions in addition to telling the time of day. Examples of a complication are a chronograph, a power reserve indicator, an alarm and a phase of the moon indicator.

Coordinated Universal Time (UTC) - see Greenwich Mean Time

C.O.S.C. - Contrôle Officiel Suisse des Chronomètres. The official Swiss Chronometer testing institute that verifies a watch's accuracy.

Côtes de Genève - a regular wave pattern obtained by engine-turning and polishing.

Countdown - time remaining in a predefined period.

Cousu Main - hand sewn. Found on well-made watch straps

Crown -

also called a stem. A knob on the outside of the watch case used to wind the mainspring in mechanical watches. Also used to set the time, when pulled out, and for setting a watch calendar (if fitted). A screw down crown is used to make the watch more water resistant and to help keep out dust.

Crystal - a transparent cover that protects the watch face. Crystals are made of glass, plastic or synthetic sapphire. Non-reflective coating on some crystals prevents glare.

        Plastic Crystals -     (acrylic) soft and flexible so they resist small impacts.
                                            Surface scratches can be buffed out.
        Mineral Crystals -    heat-hardened glass about 10 times harder than plastic.
                                             Extremely scratch resistant but must be replaced if they do scratch.
        Sapphire Crystals - 2 - 3 times harder than mineral glass and virtually
                                             scratchproof. They are quite brittle so are more likely to crack
                                             or shatter than mineral.
                                             Replacement cost is substantially higher than for mineral crystals.

Cyclops -

a small lens on the crystal to magnify the date.