The fifth is almost always a dactyl, and last must be a spondee.
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These equivalences allow for easy substitutions of one foot by another e.g. a spondee can be substituted for a dactyl.
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The sixth foot is either a spondee or a trochee.
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The final foot is a spondee.
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It is basically an iambic trimeter but with a surprise ending in the last foot, featuring a trochee or spondee that cripples the iambic rhythm.
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Neoteric poets of the late republic, such as Catullus, sometimes employed a spondee in the fifth foot, a practice Greek poets generally avoided and which became rare among later Roman poets.
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More examples
A metrical unit with stressed-stressed syllables
In poetry, a spondee is a metrical foot consisting of two long syllables, as determined by syllable weight in classical meters, or two stressed syllables, as determined by stress in modern meters. ...
In grammar, a spondee is a two-syllable word in which there is equal emphasis placed on both syllables.
Spondee is the title of a piece composed by IDM duo Matmos. It is the third track on their 2001 album "A Chance To Cut Is A Chance To Cure." The piece revolves around audio samples from a hearing test: a woman's voice reciting two-syllable words, stressing both syllables. ...
A metrical pattern characterized by two or more successively-placed accented syllables. In the following example from Shakespeare's "Othello," Othello's sleep has been disturbed by a fight. He angrily demands to know who started the fight that disturbed him. ...
A foot consisting of two accented syllables, as in the word heartbreak. In English, this foot is used occasionally, for variety or emphasis.
A two syllable foot that is comprised of two accented syllables. Usually this is done in poetry by using one syllable words (ie, rock, bird, snow) in a row. This is the opposite of the anapestic foot.
A metrical foot consisting of two accented syllables / ' ' /. An example of a spondaic word is "hog-wild." The di-spondee / ' ' ' ' / is a Classical Greek and Latin metrical foot.
In Poetry Meter, a Foot consisting of two long or stressed syllables occurring together. This form is quite rare in English Verse, and is usually composed of two monosyllabic words. ...