A line of iambic trimeter runs as follows: In this scheme, there are three ancepssyllables, marked by the symbol x. These may be long or short. Porson's Law states that, if the third anceps is long and followed by a word break, then it must be a monosyllable. A simpler summary of the Law is provided in W. W. Goodwin's Greek Grammar: M. L. West states it slightly differently, to take account of a rare situation not accounted for by Porson, where the word-break is followed rather than preceded by a monosyllable : These formulations avoid the difficulty that the reference to the ninth syllable would be inaccurate if there is resolution earlier in the line.
An example
There are, as West observes, very few breaches of Porson's Law in extant Greek tragedy. When the manuscript tradition, therefore, transmits a line that breaches Porson's Law, this is taken as a reason for suspecting that it may be corrupt. For example, the first line of Euripides' Ion, as transmitted in the mediaeval manuscript Laurentianus 32.2, the main source for the play, reads: As Porson himself had already observed in his note on line 347 in his first edition of Euripides' Hecuba, this line is irregular, since -τοις in νώτοις is long, occurs at the third anceps, and is followed by word break; it therefore breaks the law which Porson later formulated, and it is unlikely that Euripides wrote it as it stands. That the manuscript tradition is incorrect happens to be confirmed by a quotation of this line in a fragmentary papyrus of Philodemus. Philodemus' exact original text is uncertain, but it is reconstructed by Denys Page to read ὁ χαλκέοισι οὐρανὸν νώτοις Ἄτλας, which does not break Porson's Law, and therefore may be the correct text. However, other scholars have suggested various other possibilities as to what Euripides may originally have written.
Other similar laws
Several other similar laws or tendencies, such as Knox's Iamb Bridge, Wilamowitz's Bridge, Knox's Trochee Bridge, and the law of tetrasyllables, have been discovered since Porson's time. These laws apply to different styles or periods of iambic-trimeter writing. Details of these and other constraints on the trimeter are given in a 1981 article by A.M. Devine and L.D. Stephens.
Possible explanations
Similar laws which have been discovered in the dactylic hexameter are that if a word ends the fifth or fourth foot it is almost never, or only rarely, a spondee. The philologist W. Sidney Allen suggested an explanation for all these laws in that it is possible that the last long syllable in any Greek word had a slight stress; if so, then to put a stress on the first element of the last iambic metron, or the second element of the 4th or fifth dactylic foot in a hexameter, would create an undesirable conflict of ictus and accent near the end of the line. An alternative hypothesis, supported by Devine and Stephens in their book The Prosody of Greek Speech, is that in certain contexts some long syllables in Greek had a longer duration than others, and this may have made them unsuitable for the anceps position of the third metron of a trimeter.
In the Latin equivalent of the iambic trimeter, the iambic senarius, Porson's law is not observed, and lines like the following which break Porson's law are perfectly possible: Lines like the following, where there is an apparent conflict between ictus and word-stress in the last metron, are also common:
