The Heroic Couplet
Examples are from John Dryden's Absalom and Achitophel: A Poem.
1. Heroic couplets are rhymed iambic pentameter:
 ~  /
 ~  /
 ~  /
 ~  /
 ~  /
~  / 
~  / 
~  / 
~  / 
~  / 
In pi /
ous times /
ere priest /
craft did
begin
Before
poly /
gamy /
was made /
a sin
The accent or stress pattern (~ / ) may vary (slightly or greatly). Such variation is worth noticing:
/       /
         
Plots, true/
or false,/
are ne/
cesa/
ry things,
 
To raise/
up com/
monwealths/
and ru/
in kings.
 (ll. 83-4)
2. The mid-line pause (caesura) usually occurs after the fourth syllable:
 
~      / 
~      / 
~      / 
~      / 
~      / 
 
Else why/ 
should he //
with wealth/
and hon/
our blest,
 
~      / 
~      / 
~      / 
~      / 
~      / 
 
Refuse/
his age //
the need/
ful hours/
of rest?
(ll. 165-6)

When this regular pattern is followed, the first half of the line has four syllables altogether, two of them accented; the second half has six syllables, three of them accented. The second half of the line is therefore potentially more emphatic.
 

3. The first line of a couplet generally holds its own to some degree (has a certain stability) and, at the same time, works with, and is completed by, the second line. Stability of the line is created in a number of ways--

a) by grammatical completeness or apparent completeness:

        Of these the false Achitophel was first;

          A name to all succeeding ages curst: (ll. 150-1)

b) or by inversion of ordinary word order (especially subject/verb/object-complement order):

            What millions has he pardoned of his foes

            Whom just revenge did to his wrath expose? (ll. 123-4)

            His courage foes, his friends his truth proclaim;

            His loyalty the king, the world his fame (ll. 357-8)

c) Sometimes the first line sets up two balanced elements that are obviously incomplete and the second line completes them and provides the stability for the couplet:

         Thus formed by nature, furnished out with arts,

            He glides unfelt into their secret hearts. (ll. 692-3)
 

4. The second line elaborates and often significantly modifies the meaning of the first:

         What cannot praise effect in mighty minds,

            When flattery soothes and when ambition blinds! (ll. 303-4)

            Yet O! that I alone could be undone,

            Cut off from empire, and no more a son! (ll. 702-3)
 

5. Chiasmus (derived from chaismos, which means placing crosswise ) is a balanced arrangement of the four basic line elements of a couplet. The first example is from Pope's The Rape of the Lock.

                            A                         B

            The hungry judges // soon the sentence sign

                            B*                         A*

            And wretches hang // that jury-men may dine.   (III, 21-2).

Dryden's use of chiasmus in A & A* is far less prominent than Pope's. What do you think of the following example?

      This moving court, that caught the people's eyes,

      And seemed but pomp, did other ends disguise: (ll. 739-40)
 

Dryden seems more likely to use a chiastic pattern over a verse paragraph. See #9.
 

6. Enjambment is the running over of the first line into the second:

         Then Israel's monarch after Heaven's own heart

            His vigorous warmth did variously impart

      To wives and slaves; and, wide as his command,

            Scattered his Maker's image through the land. (ll. 7-10)
 

7. Zeugma (literal meaning = yoke) is a figure of speech in which one word (often a verb or an adjective) is yoked (through syntax) with two words: one verb with two subjects, one adjective with two nouns. This figure of speech often creates a sense of comic junction or ironic disjunction.

         When nature prompted, and no law denied,

      Promiscuous use of concubine and bride; (ll. 4-5).
 

8. Parallelism/antithesis:

The heroic couplet is suited to the task of stressing simi- larities and differences: of balancing essentially conflicting elements or of making distinctions between apparently similar elements.

         Believe me, royal youth, thy fruit must be

      Or gathered ripe, or rot upon the tree. (ll. 250-1)

         Your case no tame expedients will afford:

      Resolve on death, or conquest by the sword. (ll. 455-6)
 

9. Couplet "mechanics" may be "spread" over several lines, creating verse paragraphs or longer analytical units.

         What cannot praise effect in mighty minds,

           When flattery soothes and when ambition blinds!

           Desire of power, on earth a vicious weed,

           Yet, sprung from high, is of celestial seed:

           In God 'tis glory; and when men aspire,

           'Tis but a spark too much of heavenly fire.

           The ambitious youth, too covetous of fame,

           Too full of angels' metal in his frame,

           Unwarily was led from virtue's ways,

           Made drunk with honour, and debauched with praise.
 

10. Variation of regular patterns: All regular features--e.g., stress pattern, position of mid-line pause--can be varied in order to work in various ways against the reader's expectation.
 
 
 

References:

Wallace C. Brown, The Triumph of Form.

William Bowman Piper, The Heroic Couplet.

Ruth C. Wallerstein, "Development of the Rhetoric and Metre of the Heroic Couplet, Especially in 1625-45. PMLA, March 1935, pp. 166-209.

George Williamson, "The Rhetorical Pattern of Neo-classical Wit," Modern Philology, vol. 33, August 1935, 55-81.