Mechanical wristwatches are made of parts, small parts, and a lot of them. Even the simplest mechanical watch is comprised of a surprising number of parts, none of which are superfluous.

While servicing an Audemars Piguet caliber 2120, I thought it would be interesting for the average, non-technical watch owner to see an inventory of the parts in a simple, high-grade watch. The caliber 2120 is a 12.5”’ (28 millimeter), 2.45 millimeter thick, automatic winding movement with hours and minutes display. It runs at 19,800 beats per hour, utilizing 36 jewels.

The parts comprising the watch are (1) the winding rotor; (2) rotor retention clip; (3) rotor bearing; (4) rotor bearing retention ring; (5) rotor support ring; (6, 7, 8, 9) rotor rim screws; (10) 21K rotor rim; (11) mainspring barrel bridge; (12) barrel bushing; (13) ball-bearing winding transfer gear outer race; (14) ball-bearing winding transfer gear inner race; (15) retention screw; (16) ball bearings (O.K. there are six!); (17) switching rocker upper jewel; (18) barrel click upper jewel; (19) reduction wheel upper jewel; (20) hand wind-disconnect wheel upper jewel; (21) switching rocker lead wheel; (22) switching rocker lower plate;

(23) switching rocker following wheel; (24) switching rocker upper plate; (25) first winding reduction wheel; (26) first winding reduction wheel pinion; (27) first winding transfer wheel; (28) first winding transfer wheel pinion; (29) hand wind disconnect wheel lower plate; (30) hand wind disconnect wheel upper plate; (31) hand wind disconnect wheel sliding pinion upper section; (32) hand wind disconnect wheel sliding pinion lower section; (33) hand wind disconnect wheel sliding pinion spring; (34) crown wheel ring; (35) setting lever screw; (36) barrel ratchet click; (37) crown wheel center; (38-41) barrel bridge screws; (42-47) crown and ratchet wheel screws; (48) ratchet wheel.

(49) Automatic winding click bridge; (50) automatic winding click; (51) click bridge screw; (52-55) rotor peripheral roller plates; (56-59) rotor peripheral roller jewels; (60-63) rotor peripheral roller axles; (64-71) rotor peripheral roller plate screws.

(72) mainspring barrel; (73) barrel cover; (74) barrel arbor lower section; (75) barrel arbor upper section; (76-78) barrel arbor screws; (79) vertical transfer automatic winding gear upper section; (80) vertical transfer automatic winding gear lower section; (81) vertical transfer automatic winding gear screw.

(82) Winding-setting stem; (83) crown O-ring; (84) crown; (85) clutch lever with integrated spring; (86) clutch; (87) winding pinion; (88) cannon pinion; (89) hour wheel; (90) setting lever; (91) hand-setting intermediate wheel; (92) hand setting second intermediate wheel; (93) minute wheel; (94) keyless works cover; (95-96) keyless works cover screws.

(97) Balance wheel; (98-103) balance adjustment weights; (104) balance spring stud; (105) balance spring stud upper section; (106-107) balance spring stud screws; (108) shock protection block; (109) shock protection bowl; (110) shock protection spring; (111) balance staff; (112) balance impulse roller; (113) impulse roller jewel; (114) balance safety roller; (115) balance spring and collet; (116) shock protection olive jewel; (117) shock protection cap jewel; (118) balance cock screw.

(119) Movement main plate; (120) lower balance shock housing; (121) lower balance shock bowl; (122) lower balance shock spring; (123) lower balance olive jewel; (124) lower balance cap jewel; (125) escape wheel olive jewel; (126) escape wheel cap jewel and retention spring; (127) fourth wheel lower olive jewel; (128) third wheel lower olive jewel; (129) center wheel lower olive jewel; (130) automatic winding vertical transfer wheel lower jewel; (131) switching rocker lower jewel; (132) winding click lower jewel.

(133) Automatic winding first transfer wheel lower jewel; (134) automatic winding reduction wheel lower jewel; (135) hand wind disconnect wheel lower jewel; (136) winding click spring; (137) pallet lever bridge; (138-139) pallet lever bridge screws; (140) pallet lever; (141) pallet lever safety pin; (142) exit pallet jewel; (143) entry pallet jewel; (144-145) dial

(146) escape and fourth wheel bridge; (147) escape wheel upper olive jewel; (148) escape wheel upper cap jewel; (149) escape wheel upper cap jewel retention spring; (150-151) escape and fourth wheel bridge screws; (152) center wheel; (153) center wheel pinion; (154) third wheel; (155) third wheel pinion; (156) fourth wheel; (157) fourth wheel pinion; (158) escape wheel; (159) escape wheel pinion; (160-161) movement case lugs; (162-162) case lug screws; fourth wheel upper olive jewel.

(165) The mainspring; and (166) mainspring slip bridle.

The (167) dial; (168) hour hand; and (169) minute hand.

The watch is completed with (170) the case band; (171) case bezel; (172) case bezel gasket; (173) crystal; (174) crystal gasket; (175) the case back; (176) case back gasket; (177) rear crystal; (178) rear crystal gasket.