4.3.4 - Implied Offbeat Rule
"An offbeat may be implied between two stressed syllables" (REP 174).
Attridge places this rule last among the deviation rules because it most disrupts metrical regularity. Since the perception of meter particularly depends on where the stressed syllables fall, a sequence of stressed syllables is especially disruptive of the metrical set. Hence, this rule specifies that if stressed syllables fall consecutively, they will be separated by an implied, rather than actual offbeat. In other words, there is no syllable that actually corresponds to an implied offbeat. It is rather a notional pause between the stressed syllables that reflects again the naturally alternating pattern of stressed and unstressed syllables. Because the occurrence of consecutive stresses is so disruptive, the implied offbeat is always accompanied by additional compensatory action within the line, namely a double offbeat (two offbeats in a row) immediately before or after the implied offbeat (but not at the very end of the line).
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More than one set of implied + double offbeats may occur in some lines like this famous example from Andrew Marvell's "The Garden" (note that the line is in iambic tetrameter rather than pentameter). While this line has given many critics a great deal of trouble to explain, in Attridge's system, it is a straightforward, though doubled example of the implied offbeat and its conditions. You may want to think about how disruptive this line is to your perception of the meter; it will help to read the whole poem (see PIE 318-320, line 48).
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Other poets--Donne is a prime example, vary the conditions of the implied offbeat and place the compensatory double offbeat at some distance from the implied offbeat. This variation pushes the reader's sense of regular metricality to the very edge and is one of the most disruptive variations you will see. Here is a much-discussed example of this pattern from Keats:
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Mark the s's, w's, beats and offbeats, including implied and double offbeats in the following:
Which yet survive, stamped on these lifeless things,
Both the year's and the day's deep midnight is