• A short syllable contains a short quantity vowel, such as the nominative singular ending of the first declension: nauta.

 
• A long syllable contains a long quantity vowel, such as the ablative singular of the first declension: nautâ.

 
• A long syllable may also contain a diphthong, or two vowels pronounced together, such as the genitive singular of the first declension: nautae.  (The -au- in nauta is also considered long under this rule.)

In all of the above examples, the syllables are short or long by nature, and must be remembered as such during the scanning process.

Here are some general rules for select vowels and syllables:

• A final -o, -i, or -u is usually long by nature.

common exception: the final -i in tibi and mihi, which may be either short or long as the meter requires. (The first -i- in both words is always short.)

• A final -as, -es, or -os is usually long by nature.

  common exceptions: Greek proper names and other Greek words taken directly into Latin.

• A final -a or -is is often short by nature.

  common exceptions: -a in the first declension ablative singular is always long, as is -is in the first and second declensions ablative / dative plural.

• A final -e is usually short by nature.

common exceptions: -e in the fifth declension ablative singular; -e on adverbs formed from second declension adjectives.

• A final -us is usually short by nature.

common exceptions: fourth declension genitive singular, and nominative / accusative plural (but not fourth declension nominative singular).

• A final -am, -em, or -um is always short by nature.