R

Rattrapante – (German: Doppelchrono) also Split Seconds  -  2 seconds hands on a chronograph.  When a button is pushed, the 2 hands start together and move in unison.  When the button is pushed a second time one of the hands stops to allow the time to be noted and the other seconds hand continues to time the event, thus “splitting” the hands.  A third push of the button makes the seconds hand that was stopped “catch up” to the hand that is still moving and once again the 2 hands move in unison. 
                    Example: You want to time how long it takes a racing car to complete 4 laps but you also want to record the time at the end of each lap.  At the end of each lap push a button and stop one of the seconds hands to record the time.  The other seconds hand continues timing.  Another push of the button starts the hands moving in unison once again.

Refining - eliminating impurities in metals.

Regulating elements - set of parts comprising the regulating system (spring balance) and the escapement (escape wheel, lever and roller).

Regulator - part of the watch movement that makes the time more accurate by speeding up or slowing down the beats.

Repeater - The time is indicated by sound.  Chimes ring out the time when a button is pressed. Different sounding chimes are used for the hours, quarter hours and minutes.

        e.g.      Hour =                  Ding
                   Quarter Hour =     Ding Dong
                   Minutes =              Dong

                    Ding Ding Ding - Ding Dong - Dong Dong Dong Dong
                    indicates that the time is 3:19                

Reserve de Marche - see Power Reserve

Retrograde hands -

hands that move backwards to measure a new period of time.
On this watch when the hand reaches the 60 minute position it jumps back to zero and the arc starts over again.
 

Reverso - a rectangular wrist watch that can be turned over within the main frame. One side shows one dial and the other side shows another dial. Could be used for displaying dual-time.

Rolled Gold - a thin sheet of gold bonded to a base metal.

Roman Calendar - the calendar in use in ancient Rome until 46 B.C., when it was replaced with the Julian calendar.
 

Rotor – The part of an automatic watch that winds the mainspring. A flat piece of metal swivels on a pivot with the motion of the wearer's arm. It's rotations continually wind the mainspring of the watch. It  turns freely in both directions and uses the force of gravity to wind the mainspring. Some rotors only turn in one direction or less than 360°. Each time it moves, it advances the ratchet-wheel, which keeps the spring wound.

Ruby - Very hard stone, usually synthetic, that prevents the wear of the gear-train parts.