The basic unit in Greek and Latin prosody is a mora, which is defined as a single short syllable. A long syllable is equivalent to two morae. A long syllable contains either a long vowel, a diphthong, or a short vowel followed by two or more consonants. Various rules of elision sometimes prevent a grammatical syllable from making a full syllable, and certain other lengthening and shortening rules (such as correption) can create long or short syllables in contexts where one would expect the opposite. The most important Classical metre is the dactylic hexameter, the metre of Homer and Virgil. This form uses verses of six feet. The word "dactyl" comes from the Greek word "daktylos" meaning "finger", since there is one long part followed by two short stretches. The first four feet are dactyls ("daa-duh-duh"), but can be spondees ("daa-daa"). The fifth foot is almost always a dactyl. The sixth foot is either a spondee or a trochee ("daa-duh"). The initial syllable of either foot is called the "ictus", the basic "beat" of the verse. There is usually a caesura after the ictus of the third foot. The opening line of the "Æneid" is a typical line of dactylic hexameter:  In this example, the first and second feet are dactyls; their first syllables, "Ar" and "rum" respectively, contain short vowels, but count as long because the vowels are both followed by two consonants. The third and fourth feet are spondees, the first of which is divided by the main caesura of the verse. The fifth foot is a dactyl, as is nearly always the case. The final foot is a spondee. The dactylic hexameter was imitated in English by Henry Wadsworth Longfellow in his poem "Evangeline": Notice how the first line:  Follows this pattern:  Also important in Greek and Latin poetry is the dactylic pentameter. This was a line of verse, made up of two equal parts, each of which contains two dactyls followed by a long syllable, which counts as a half foot. In this way, the number of feet amounts to five in total.