Wearing a pocket watch sans waistcoat

[RB51]

A bit of a deviation on this thread, if Macman will permit, but is there any convention on the wearing of a pocket watch with a waistcoat? In some pictures I've noticed that the chain only loops from the button hole to one side of the waistcoat, and at other times it starts in one pocket, loops through the button hole and then onto the other side. And pardon my ignorance, but what are the fobs for? Are they usually purely decorative, or might they have some meaning to the wearer?

Thanks for any enlightenment.

Laurie