Overview
Systems of scansion, and the assumptions (often tacit or even subconscious) that underlie them, are so numerous and contradictory that it is often difficult to tell whether differences in scansion indicate opposed metrical theories, conflicting understandings of a line's linguistic character, divergent practical goals, or whether they merely constitute a trivial argument over who has the "better ear" for verse.
To understand any form of scansion, it is necessary to appreciate the difference between meter and rhythm.
The rhythm of language is infinitely varied; all aspects of language contribute to it: loudness, pitch, duration, pause, syntax, repeated elements, length of phrases, frequency of polysyllabic words... As C.S. Lewis observes, "f the scansion of a line meant all the phonetic facts, no two lines would scan the same way".
Meter is another matter. It is an ordering of language by means of an extremely limited subset of its characteristics. In English (and in many modern languages) the language is ordered by syllabic stress. All other aspects of language are present, indeed they are vital to the rhythm of the verse; but they are not ordered by the meter.
However, marking stress is not the same as marking meter. A perfectly regular line of iambic pentameter may have anywhere from 2 to 9 stresses, but it is still felt to exhibit 5 pulses or beats. This can most easily be understood through the principle of relative stress: an unstressed syllable between 2 even slightly weaker syllables may be perceived as a beat; and the reverse is true of a stressed syllable between 2 even slightly stronger syllables. These phenomena are called "promotion" and "demotion". Thus a syllable, regardless of its level of stress, that realizes a beat is ictic; and a syllable, regardless of its level of stress, that does not is nonictic. Ictus refers to the position within a line that is experienced as a beat, or to the syllable that fills it.
T.V.F. Brogan issues a stern warning about the temptations of overly detailed scansion:
Since meter is a system of binary oppositions in which syllables are either marked or unmarked (long or short; stressed or unstressed), a binary code is all that is necessary to transcribe it. . . . It is natural to want to enrich scansion with other kinds of analyses which capture more of the phonological and syntactic structure of the line . . . But all such efforts exceed the boundary of strict metrical analysis, moving into descriptions of linguistic rhythm, and thus serve to blur or dissolve the distinction between meter and rhythm. Strictly speaking, scansion marks which syllables are metrically prominent -- i.e. ictus and nonictus -- not how much. Scansions which take account of more levels of metrical degree than two, or intonation, or the timing of syllables are all guilty of overspecification.
Prosodists seldom explicitly state what they are marking in their scansions. For clarity, scansions that mark only ictus and nonictus will be called "metrical scansions", and those which mark stress or other linguistic characteristics will be called "rhythmic scansions".
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