Mora (plural moras or morae) is a unit of sound used in phonology that determines syllable weight (which in turn determines stress or timing) in some languages. Like many technical linguistics terms, the exact definition of mora is debated. The term comes from the Latin word for "linger, delay", which was also used to translate the Greek word chronos (time) in its metrical sense.
A syllable containing one mora is said to be monomoraic; one with two moras is called bimoraic.
In general, moras are formed as follows:
1. A syllable onset (the first consonant(s) of the syllable) does not represent any mora.
2. The syllable nucleus represents one mora in the case of a short vowel, and two moras in the case of a long vowel or diphthong. Consonants serving as syllable nuclei also represent one mora if short and two if long. (Slovak is an example of a language that has both long and short consonantal nuclei.)
3. In some languages (for example, Japanese), the coda represents one mora, and in others (for example, Irish) it does not. In English, it is clear that the codas of stressed syllables represent a mora (thus, the word cat is bimoraic), but it is not clear whether the codas of unstressed syllables do (the second syllable of the word rabbit might be monomoraic).
4. In some languages, a syllable with a long vowel or diphthong in the nucleus and one or more consonants in the coda is said to be trimoraic (see pluti).
In general, monomoraic syllables are said to be light syllables, bimoraic syllables are said to be heavy syllables, and trimoraic syllables (in languages that have them) are said to be superheavy syllables. Most linguists believe that no language uses syllables containing four or more moras.
Japanese is a language famous for its moraic qualities. Most dialects including the standard use moras (in Japanese, onji) as the basis of the sound system rather than syllables. For example, haiku in modern Japanese do not follow the pattern 5 syllables/7 syllables/5 syllables, as commonly believed, but rather the pattern 5 moras/7 moras/5 moras. As one example, the Japanese syllable-final n is moraic, as is the first part of a geminate consonant. For example, the word NIPPON ('Japan' in Japanese) has four moras (NI-P-PO-N)
See Geiger p. 63 on the "Law of Mora":
In Pali, a syllable can contain only one mora or two moras but never more. The syllable is thus either 1. open with short vowel (one mora), or 2. open with long vowel (two moras), or 3. closed with short vowel (two moras). Every syllable with a nasal vowel is considered as closed. Due to this law, where Skt. has a long vowel before double-consonance, Pali has there either 1. short vowel before double-consonance or 2. long vowel with the following double-consonance simplified.
In Prakrit (Pali, etc.) there are not allowed more than two morae for one syllable. If a word has a long vowel (2 morae) and is followed by two consonants, the vowel is often shortened, i.e., Pali ettha-, etc. mārga- (Skt.) is not allowed in Pali for this reason.
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