[Mithalwen]

Keeper ... while you make some interesting points, I think you have slightly misinterpreted the expression "as a rule". In current usage, the first sense of rule that comes to mind is probably to mean a "regulation" and while the words are similar they were probably not completely interchangeable since we have the expression "rules and regulations". But the sense I think Tolkien is using it is as a measuring device - equivalent to ruler. A yard-rule was a measuring stick a yard long.

I think it may be a contraction of the old phrase " as a rule of thumb" which refers to measuring things in thumb widths on the same principal as the foot and hand (which did become "fixed" units of measure). Because thumbs vary in size but not so much that they are useless as a term of reference, the expression came to mean "usually" as SpM says. It implies that the information is a general guide rather than absolutely precise.

In the phrases Elianna has quoted, I always think that Tolkien sounds a bit like a guidebook. A reason maybe he usually uses it for hobbits may be becuse they are a "strange" species. He invented them and he needs to describe them whereas most readers (at least English ones) would have an idea about what dwarves and wizards look like. Elves are more problematic because of the confusion with "itty bitty fairies"

This kind of expression has probably fallen out of use somewhat since the modern world requires us to be more specific rather in the way time was standardised with the development of railway networks.