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 meter as defined by the Alexandrian grammarians was the dactylic hexameter, the meter of Homeric poetry. This form uses verses of six feet. The first four feet are normally dactyls, but can be spondees. The fifth foot is almost always a dactyl. The sixth foot is either a spondee or a trochee. 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.