9101.21 - With automatic winding:

This HTS classification, 9101.21, specifically covers wrist watches that feature automatic winding mechanisms and have cases made of precious metal or metal clad with precious metal. The defining characteristic here is the self-winding capability, where the watch's movement is powered by the wearer's motion, eliminating the need for manual winding. These timepieces are a subset of the broader category of precious metal cased wrist watches.

Distinguished from its sibling category, 9101.29 "Other," this sub-heading focuses exclusively on the presence of an automatic winding mechanism. Category 9101.29 would encompass all other precious metal cased wrist watches that do not possess this automatic winding feature, such as those requiring manual winding or powered by battery.

Within this category, further subdivision is made based on the complexity of the movement. Subcategory 9101.21.80 "Other" would then classify automatic winding, precious metal cased wrist watches based on criteria not related to the winding mechanism itself, such as the number of jewels in the movement, which is a key indicator of mechanical watch quality and precision.