Porson's Law

Porson's Law, or Porson's Bridge, is a metrical law that applies to iambic trimeter, the main spoken metre of Greek tragedy. It does not apply to iambic trimeter in Greek comedy. It was formulated by Richard Porson in his critical edition of Euripides' Hecuba in 1802.^[1]

The law states that if a non-monosyllabic word ends on the 9th element of an iambic trimeter, the 9th element must be a short syllable.