A heroic couplet is a traditional form for English poetry, commonly used for epic and narrative poetry; it refers to poems constructed from a sequence of rhyming pairs of iambic pentameter lines. Use of the heroic couplet was first pioneered by Geoffrey Chaucer in the Legend of Good Women and the Canterbury Tales.
- O could I flow like thee, and make thy stream
- My great example, as it is my theme!
- Though deep yet clear, though gentle yet not dull;
- Strong without rage, without o'erflowing full.
The term "heroic couplet" is sometimes reserved for couplets that are largely closed and self-contained, as opposed to the enjambed couplets of poets like John Donne. The heroic couplet is often identified with the English Baroque works of John Dryden and Alexander Pope, who used the form for their translations of the epics of Virgil and Homer, respectively. Major poems in the closed couplet, apart from the works of Dryden and Pope, are Samuel Johnson's The Vanity of Human Wishes, Oliver Goldsmith's The Deserted Village, and John Keats's Lamia. The form was immensely popular in the 18th century. The looser type of couplet, with occasional enjambment, was one of the standard verse forms in medieval narrative poetry, largely because of the influence of the Canterbury Tales.
English heroic couplets, especially in Dryden and his followers, are sometimes varied by the use of the occasional alexandrine, or hexameter line, and triplet. Often these two variations are used together to heighten a climax. The breaking of the regular pattern of rhyming pentameter pairs brings about a sense of poetic closure. Here are three examples from Book IV of Dryden's translation of the Aeneid.
- Nor let him then enjoy supreme command;
- But fall, untimely, by some hostile hand,
- And lie unburied on the barren sand!
- (ll. 890-892)
- Her lofty courser, in the court below,
- Who his majestic rider seems to know,
- Proud of his purple trappings, paws the ground,
- And champs the golden bit, and spreads the foam around.
- (ll. 190-193)
Alexandrine and Triplet 
- My Tyrians, at their injur’d queen’s command,
- Had toss’d their fires amid the Trojan band;
- At once extinguish’d all the faithless name;
- And I myself, in vengeance of my shame,
- Had fall’n upon the pile, to mend the fun’ral flame.
- (ll. 867-871)
Modern Use 
Twentieth century authors have occasionally made use of the heroic couplet, often as an allusion to the works of poets of previous centuries. An example of this is Vladimir Nabokov's novel Pale Fire, the second section of which is a 999 line, 4 canto poem largely written in loose heroic couplets but also allowing for frequent enjambment. Here is an example from the first canto.
- And then black night. That blackness was sublime.
- I felt distributed through space and time:
- One foot upon a mountaintop. One hand
- Under the pebbles of a panting strand,
- One ear in Italy, one eye in Spain,
- In caves, my blood, and in the stars, my brain.
- (Canto One. 147-153)
- Hobsbaum, Philip. Metre, Rhythm and Verse Form. Routledge (1996) p.23
- Ferrando, Ignasi Navarro. In-roads of Language: Essay in English Studies. Universitat Jaume I(1996) p.125