Oh and you don't get more than one truck on a mast - and you don't get that until you get to the very top of the topgallant or royal mast. The bit you are referring to is called the 'cap'. So in the maintop (where the mainmast joins with the main topmast) the mainmast ends in a cap. Of course the mainmast can refer to the bottom bit of the stick in the middle, or to the whole mast, topmast, topgallant mast and royal mast together.

Yes I know I could bore for England on this subject...