4.3.2 - Promotion Rule
"An unstressed syllable may realize a beat when it occurs between two unstressed syllables, or with a line-boundary on one side and an unstressed syllable on the other" (REP 359).
This rule reflects the tendency in English for a basically alternating rhythm, as we saw in the Rhythm Rule (e.g., Tennesee and Tennesee walking horse). Syllables are promoted when there would otherwise be three consecutively unstressed syllables in a row. That includes line beginnings and endings since the perceived continuity between lines extends the effective environment for unstressed syllables. Another way to think of this rule is to say that "an unstressed syllable may realize a beat when it is not adjacent to a stress in the same line" (REP 168).
(15)
Mark the stress and the metrical beats and offbeats in the following:
And burn the long-lived phoenix in her blood;