Boston Tin

Part one of a two-part, or Boston Shaker kit, the Boston Tin resembles a milkshake can and is generally made from polished stainless steel.  Occasionally it is made from sterling silver.  The tin fits over the mixing glass (part two) of the Boston kit and should be a snug fit.
 
With a gentle tap, the tin and glass should form a vacuum seal in which you can happily shake away without the fear of being drenched by a Ramos.  Without this vacuum seal, the tin and glass can separate, fly off, and injure your friends, customers and pride.  Therefore, ensure that you shake with the mixing glass facing away from your fans, or fancy expensive mirrors.