There is a final rule that often circumvents the need for memorizing vowel quantities: 

• A vowel is considered long by position when directly followed by two consonants, whether in the same word or beginning the subsequent word. 

For instance, if the nominative nauta immediately precedes the verb scit, the final -a becomes long for the purpose of filling out the meter: nautâ scit. (The word is still nominative, of course.) 

This rule is not ironclad, as certain consonant combinations—like -cr, -pr, and -tr—will not always "make position."

Some important points.

• The consonant -h- is not usually considered a full consonant, and will not cause a vowel to make position. For instance, in the phrase tenet haec ("she understands these things"), the syllable -et in tenet is short, not long.
  
  
• The consonants -x- and -z- make position all by themselves, since they are actually double consonants (-ks- and -ds-, respectively).
  
  
• The combination -qu-, which is pronounced -kw-, just as in English, is treated as a single consonant, and never makes position. Furthermore, the -u- never receives a long or short mark during scansion, as is the practice for consonants.
  
  
• The vowel -i- is sometimes considered a consonant, usually when it begins a word and is followed by a vowel. In such cases, e.g. Iuno, the -i- is pronounced as an English -y- (which, recall, is likewise sometimes a vowel, sometimes a consonant).

  A good rule of thumb is to find a derivative from the word in question: if the -i- becomes a -j- in English (e.g. Juno > Iuno), then it is a consonant, and may help another word make position. Furthermore, the -i- in question should not receive a long or a short mark.