Wikipedia
Dactylic pentameter is a form of meter in poetry. The dactyl, which is made of a stressed (or long) syllable followed by two unstressed (or short) syllables, is repeated five times to create a pentameter line. In modern poetry, a simple form of dactylic pentameter can be seen in Stan Galloway's poem Angels' First Assignment, the first two lines of which read:
"Are you still standing there east of the Garden of Eden, or
were you relieved by the flood that revised our geography?"
In classical literature, it is usually found in the second line of the classical Latin or Greek elegiac couplet, following a dactylic hexameter.
The meter consists of two halves, both shaped around the dactylic hexameter line up to the main caesura. That is, it has two dactyls (for which spondees can be substituted), following by a longum, followed by two dactyls (which must remain dactyls), followed by a longum. Thus the line most normally looks as follows (note that — is a long syllable, ∪ a short syllable and U either one long or two shorts):
— U | — U | — || — ∪ ∪ | — ∪ ∪ | —As in all classical verse forms, the phenomenon of brevis in longo is observed, so the last syllable can actually be short or long. Also, the line has a diaeresis, a place where word-boundary must occur, after the first half-line, here marked with a ||.
"Pentameter" is a slightly strange term for this meter, as it seems to have six parts, but the reason is that the two halves of the line, broken here by the ||, each have two and a half feet. Two and a half plus two and a half equals five. The two half-lines are each called a hemiepes (half-epic), as they resemble half a line of epic dactylic hexameter.
The pentameter is notable for its very structured quality: no substitutions are allowed except in the first two feet.