The Allen and Greenough is still under construction;
so some links may not work quite the way you would expect.
602. The poetry of the Indo-European people seems
originally to have been somewhat like our own, depending on accent for its
metre and disregarding the natural quantity of syllables. The Greeks,
however, developed a form of poetry which, like music, pays close
attention to the natural quantity of syllables; and the Romans borrowed
their metrical forms in classical times from the Greeks. Hence Latin
poetry does not depend, like ours, upon accent and rhyme; but is measured,
like musical strains, by the length of syllables. Especially does it
differ from our verse in not regarding the prose accent of the words, but
substituting for that an entirely different system of metrical accent or
ictus (see § 611. a). This depends upon the character
of the measure used, falling at regular intervals of time on a long
syllable or its equivalent. Each syllable is counted as either long or
short in Quantity; and a long syllable is generally reckoned equal in
length to two short ones (for exceptions, see § 608. c-e).
The quantity of radical (or stem) syllables - as of short a in
pater or of long a in máter - can be learned only by
observation and practice, unless determined by the general rules of
quantity. Most of these rules are only arbitrary formulas devised to
assist the memory; the syllables being long or short because the
ancients pronounced them so. The actual practice of the Romans in
regard to the quantity of syllables is ascertained chiefly from the usage
of the poets; but the ancient grammarians give some assistance, and in
some inscriptions long vowels are distinguished in various ways, - by
the apex, for instance, or by doubling (§ 10. e. N.).
Since Roman poets borrow very largely from the poetry and mythology of
the Greeks, numerous Greek words, especially proper names, make an
important part of Latin poetry. These words are generally employed in
accordance with the Greek, and not the Latin, laws of quantity. Where
these laws vary in any important point, the variations will be noticed in
the rules below.