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The picture shows a truss of a lower yard. In the middle of the yard is a
steel band with an eye on which a hanger chain is fixed. So the lower yard
is hanging on a chain, not on the truss itself. The lower topsail yard has a
same kind of truss, but without a hanger chain. If it needs extra support, it
stands on an iron or steel rod.
The parral is being employed at travelling yards, like the upper topsail yard.
So the parral slides up and down the topmast.
There are more types of parrals (see Middendorf's book), which I translate
as a cleatparral, a cradleparral or a cockparral (it might not even be English).
But the parral mentioned above is the most common. For the calculations it makes no
difference, as long as you keep in mind where the turning point lies.
The
truss is an arrangement which connects the yard to a mast or a top, in such
a way that the
yard can turn horizontally as well as vertically.
The truss is also meant to keep the yard at a certain distance of the mast. This increases the brace
angle, because the yard is standing further away from the shrouds.
The truss is meant for fixed yards, yards that don't slide up and down the mast, like the
lower yard.
The turning point of these yards, and that is where it's all about, lies in the
bolt in front of the mast.
The consequence of this arrangement is, that the turning point of this yard lies in
the centre of the topmast. Therefore the effect of the turning away of the yard from
the line of keel is as large, if not larger, as of the lower topsail yard (with a truss),
although the upper topsail yard with its fittings is lighter and smaller then its lower brother.
Nowadays rigs with all the yards fixed on their place are becoming common practice, a pity. 
