The heartbeat of Shakespeare and Milton: FIVE iambs, each a soft-then-STRONG pair — da-DUM da-DUM da-DUM da-DUM da-DUM — ten syllables to the line. The living line breathes by breaking the pattern (a trochee to open, a spondee for weight), but the five-beat spine holds. Slide to add substitutions and watch the beats survive the variation.
Iambic pentameter is five iambic feet — ten syllables in weak–STRONG pairs (≮—), so positions 2,4,6,8,10 carry the beat. Real verse admits substitutions (an initial trochee —≮, a spondee ——, a pyrrhic ≮≮) without losing the five-beat identity, because the metrical GRID persists under the rhythmic surface. A fail-loud self-check throws unless the base line has 10 syllables and 5 stress positions. ◆ the metrical rule is exact, node-verified.
The 10-syllable, 5-beat grid is exact; whether a given real line ‘is’ pentameter under heavy substitution is a reading — the tension between the ideal grid and the spoken line is the art, not a defect.