Pay attention now. I may ask questions later.
Masts on square riggers usually come in three parts.
The big bit, that sticks out of the deck (or floor), a slightly smaller bit is joined to the top of that bit, then another even smaller bit is stuck on top of that bit. OK so far?
The masts have names, and so too do the bits that go to make 'em.
The mast nearest the front (or bow) is called The Foremast. And that too is the name for the first bit of it that sticks up out of the deck. The next bit up on this 'ere "foremast" is called the "fore-top-gallant-mast" or (as we landlubbers wot knows a bit and wish to appear swanky calls it) fore't'gall'nt.
The next bit up from THAT is called the "Fore-top-mast" or fore-top.
Then as ye travel further back along the deck ye will find another bloody great mast. This is called The Mainmast. And guess what? The first bit of that mast also has the same name. The bit above that is called the "main-top-gallant" (main't'gall'nt) and the bit above that is called the Main-top-mast or maintop.
If this boat (ship) has only three masts, the last one is called The Mizzen-mast. Everything already said about the Fore & Main masts also applies to this mast.
If there be more than three masts, ye will have to ask someone who knows their names, cos I sure as hell dont! I believe the word Crojik is mentioned somewhere.......
Anything connected to these various bits, such as sails, lengths of wood, rope etc is also called after these bits i.e. the lump of canvas attached to the fore mast is called the fore-sail (or fore course) the canvas above that is called the fore-top-gallant-sail. If there be TWO sails on the t'gall'nt then they would be called lower and upper t'gall'nt sails accordingly. Same goes for the top bit. If there be any sail above the topsail, it be called a "royal".
Anyway, the whole point of this little lesson is this :-
The bit where the various pieces of mast meet is called a "truck"
So the main truck is the joining up bit where the main mast meets the main-top-gallant mast, see ?
(and by the way, "bits" are something else entirely)

If any of this turns out to be a complete load of bollards, then you didnt hear it from me, right!