NOT A TEACHER
(1) Maybe it might help you to remember that these "rules" are often arbitrary.
That is, over time certain people have been successful in convincing teachers
that something is correct. Then the teachers (and their books) keep repeating the
"rule" until most people accept it. In other words, when it comes to hyphens, there
may not be any really good "reason." (Before World War II, some people still
spelled "today" as "to-day." Then "someone" (important newspapers? important
writers?) decided to spell it "today." )
(2) I am looking at one of the most respected grammar books that are used by
teachers: A Comprehensive Grammar of the English Language (1985 edition).
On page 397, it says that we should say:
"a three-quarter majority" but "a two-thirds majority." It does not explain the
difference except to say "Note the [difference]"!!!
(3) And that book supports the rule that you already know about:
A ten-year-old girl. / Two-digit inflation./ A ten-dollar bill.
(4) But that rule has an exception, too:
(a) A book entitled The Grammar Book (which is used by many teachers) tells us that
the measure word "year" is singular in "It's a fifty-year-old house" because that is
the rule. But it then tells us that we should say: "Greece: A Centuries-old Framework
for Contemporary Living." Why? Because the rule only applies to specific measures
(that is, with a number). So "Five-centuries-old" would not be correct.
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