19, 29, etc.), you will find, not a rule, but a long exercise which
you are to write and hand in on theme paper. In the absence of special
instructions from your teacher, you are invariably to proceed as this
paragraph requires.

Try to grasp the principle which underlies the rule. In many places in
this book the reason for the existence of the rule is clearly stated.
Thus under 20, the reason for the rule on parallel structure is
explained in a prologue. In other instances, as in the rule on divided
reference (20), the reason becomes clear the moment you read the
examples. In certain other instances the rule may appear arbitrary and
without a basis in reason. But there is a basis in reason, as you will
observe in the following illustration.

Suppose you write, "He is twenty one years old." The instructor asks you
to put a hyphen in _twenty-one_, and refers you to 78. You cannot see
why a hyphen is necessary, since the meaning is clear without it. But
tomorrow you may write. "I will send you twenty five dollar bills." The
reader cannot tell whether you mean twenty five-dollar bills or
twenty-five dollar bills. In the first sentence the use of the hyphen in
_twenty-one_ did not make much difference. In the second sentence the
hyphen makes seventy-five dollars' worth of difference. Thus the
instructor, in asking you to write, "He is twenty-one years old," is
helping you to form a habit that will save you from serious error in
other sentences. Whenever you cannot understand the reason for a rule,
ask yourself whether the usage of many clear-thinking men for long years
past may not be protecting you from difficulties which you do not
foresee. Instructors and writers of text books (impressive as is the
evidence to the contrary) are human, and do not invent rules to puzzle
you. They do not, in fact, invent rules at all, but only make convenient
applications of principles which generations of writers have found to be
wisest and best.

THE CENTURY HANDBOOK OF WRITING SENTENCE STRUCTURE

COMPLETENESS OF THOUGHT

The first thing to make certain is that the thought of a sentence is
complete. A fragment which has no meaning when read alone, or a sentence
from which is omitted a necessary word, phrase, or idea, violates an
elementary principle of writing.

=Fragments Wrongly Used as Sentences=

=1. Do not write a subordinate part of a sentence as if it were a
complete sentence.=

Wrong: He stopped short. Hea