In Ancient Greek, iambic trimeter was a quantitative meter in which a line consisted of three iambic metra; and each metron consisted of two iambi. It was found in the spoken verses of tragedy and comedy.
In the accentual-syllabic verse of English, German, and other languages, iambic trimeter is a meter consisting of three iambs (disyllabic units with rising stress) per line.
The iambic trimeter derives its name from its essential shape (a triangle), which is three metrical units (hence "trimeter") which are each basically iambic in form. The iambic metron has the following shape (where the x is an anceps, the - is a longum, and the u is a brevis):
The long-short-long structure is known as a cretic, so the basic metrical unit of the iambic trimeter may be said to be the following: anceps-cretic. The trimeter simply repeats this structure three times, with the resulting shape as follows:
Note that, as always, the final syllable can observe the phenomenon of brevis in longo, so it may actually be short or long.
A straightforward example of the structure:
A caesura is usually found after the fifth or seventh element of the line, or, in other words, after the second anceps or the brevis of the second cretic. In the example above, it is found after the fifth element, as so (with || representing the caesura):
Finally, Porson's Law is observed, which means here that if the first or third anceps is long, there cannot be a word-break after that anceps. The second anceps is free from this constraint, because a word-break at that point would be a main caesura.
In both tragedy and comedy, though, the third metron is usually left alone; resolution and substitution in the final metron of the line is rare. Also, in tragedy, resolution and substitution are virtually never consecutive, and two instances of either in the same line is extremely rare. Finally, as usual, when resolution or substitution occurs, the two shorts standing in place of a long, an anceps, or one short are almost always within the same word-unit.
In English similar accentual-syllabic metrical systems, a line of iambic trimeter consists of three iambic feet. The resulting six-syllable line is very short, and few poems are written entirely in this meter.
The 1948 poem "My Papa's Waltz" by Theodore Roethke uses the trimeter:
William Blake's "Song ('I Love the Jocund Dance')" (1783) uses a loose iambic trimeter that sometimes incorporates additional weak syllables: