A timepiece sub-assembly includes, irreversibly fixed to a main plate and each irreversibly pre-adjusted by the irreversible securing of adjustment and/or assembly components, after the particular timepiece function thereof has been adjusted and function checked on the test bench, functional modules including a gear train module, a display module, and a time-setting module. Other mechanical sub-assemblies, each irreversibly fixed to the preceding lower order mechanical sub-assembly, include various modules including motor module, frame-mounted self-winding device module, escapement holder regulating module, motion work train, date module, and self-winding module. A mechanical movement including at least one mechanical sub-assembly of this type and a watch including a mechanical movement of this type are also disclosed. A method of assembling mechanical sub-assemblies of this type by the irreversible securing of mechanical sub-assemblies and irreversibly pre-adjusted modules is also disclosed.