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Mora (plural morae or moras) is a unit in phonology that determines syllable weight, which in some languages determines stress or timing. As with many technical linguistic terms, the definition of a mora varies. Perhaps the most succinct working definition was provided by the American linguist James D. McCawley in 1968: a mora is “something of which a long syllable consists of two and a short syllable consists of one.” 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; a syllable with two morae is said to be bimoraic. Also, in rarer cases, a syllable with three morae is said to be trimoraic.
In general, morae are formed as follows:
- A syllable onset (the first consonant or consonants of the syllable) does not represent any mora.
- The syllable nucleus represents one mora in the case of a short vowel, and two morae 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.)
- In some languages (for example, Japanese), the coda represents one mora, and in others (for example, Irish) it does not. In English, the codas of stressed syllables represent a mora (thus, the word cat is bimoraic), but for unstressed syllables it is not clear whether the codas do so (the second syllable of the word rabbit might be monomoraic).
- 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 morae.
Ancient Greek 
Ancient Greek pitch accent is placed on only one mora in a word. An acute (έ, ή) represents high pitch on the only mora of a short vowel or the last mora of a long vowel (é, eé). A circumflex (ῆ) represents high pitch on the first mora of a long vowel (ée).
In Ganda, a short vowel constitutes one mora while a long vowel constitutes two morae. A simple consonant has no morae, and a doubled or prenasalised consonant has one. No syllable may contain more than three morae. The tone system in Ganda is based on morae.
Gilbertese, an Austronesian language spoken mainly in Kiribati is a trimoraic language. The typical foot in Gilbertese contains three morae. These trimoraic constituents are units of stress in Gilbertese. These “ternary metrical constituents of the sort found in Gilbertese are quite rare cross-linguistically, and as far as we know, Gilbertese is the only language in the world reported to have a ternary constraint on prosodic word size.”
In Hawaiian, both syllables and morae are important. Stress falls on the penultimate mora, though in words long enough to have two stresses, only the final stress is predictable. However, although a diphthong, such as oi, consists of two morae, stress may fall only on the first, a restriction not found with other vowel sequences such as io. That is, there is a distinction between oi, a bimoraic syllable, and io, which is two syllables.
Japanese is a language famous for its moraic qualities. Most dialects, including the standard, use morae (in Japanese, haku (拍) or mōra (モーラ)) rather than syllables as the basis of the sound system. Writing Japanese in kana (hiragana and katakana) demonstrates the moraic system of writing; for example, in the two-syllable word mōra, the ō is a long vowel and counts as two morae. The word is written in three symbols, モーラ (corresponding here to mo/o/ra), each containing one mora.
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 morae/7 morae/5 morae.
As one example, the Japanese syllable-final n is moraic, as is the first part of a geminate consonant. For example, the Japanese name for "Japan", 日本, has two different pronunciations, one with three morae (Nihon) and one with four (Nippon). In the hiragana spelling, the three morae of Ni-ho-n are represented by three characters (にほん), and the four morae of Ni-p-po-n need four characters to be written out as well (にっぽん).
Similarly, the names Tōkyō (to-u-kyo-u とうきょう), Ōsaka (o-o-sa-ka おおさか), and Nagasaki (na-ga-sa-ki ながさき) all have four morae, even though they have two, three, and four syllables, respectively.
In India, the mora was an acknowledged phenomenon well over two millennia ago in ancient Indian linguistics schools studying the dominant scholarly and religious lingua franca of Sanskrit. The mora was first expressed in India as the mātrā.
For example, the short vowel "a" (pronounced like a schwa) is assigned a value of one mātrā, the long vowel "ā" is assigned a value of two mātrās, and the compound vowel (a Diphthong) "ai" (which is composed of two simple short vowels, namely "a"+"i", or one long and one short vowel, namely "ā"+"i") is assigned a value of two mātrās.
Sanskrit prosody and metrics have a deep history of taking into account moraic weight, as it were, rather than straight syllables, divided into "laghu" (लघु, "light") and "deergh" / "guru" (दीर्घ / गुरु, "heavy") feet based on how many morae can be isolated in each word.
Thus, for example, the word kartr, meaning "agent" or "doer", does not contain, contrary to intuitive English prosodic principles, simply two syllabic units, but contains rather, in order, a "deergh" / "guru "/ "heavy" foot and a "laghu" / "light" foot. The reason is that the conjoined consonants 'rt' render the normally light 'ka' syllable heavy.
In the Tamil language அளபு (Alabu) is a very well defined concept that dates back to Tolkappiyam. Tolkappiyam defines short vowels as having 1 mora (ஓர் அளபு), and long vowels having 2 morae (ஈர் அளபு).
அ இ உ எ ஒ என்னும் அப் பால் ஐந்தும் ஓர் அளபு இசைக்கும் குற்றெழுத்து என்ப (Short vowels have 1 mora)
ஆ ஈ ஊ ஏ ஐ ஓ ஔ என்னும் அப் பால் ஏழும் ஈர் அளபு இசைக்கும் நெட்டெழுத்து என்ப (Long vowels have 2 morae)
மெய்யின் அளபே அரை என மொழிய (Consonants have 1/2 mora)
மெய்யொடு இயையினும் உயிர் இயல் திரியா
In Indian languages the consonant-vowel sequences are written as one unit (Abugida) but still maintain the mora of the vowel.
For example, the letter பை (Pi) = ப் (P) + ஐ (I) [consonant + long vowel] has a duration of 2 morae.
மூ அளபு இசைத்தல் ஓர் எழுத்து இன்றே. (An Alphabet/letter cannot have 3 or more morae).
நீட்டம் வேண்டின் அவ் அளபுடைய கூட்டி எழூஉதல் என்மனார் புலவர். (Increasing the duration of a letter beyond 2 morae should only be accomplished by adding an appropriate vowel). Tolkappiyam provides an example for this in the same verse with a word எ"ழூஉ"தல். ழூஉ = (ழ் + ஊ) + உ [ 2 + 1 = 3 morae]
Tolkappiyam goes on to define the duration of a mora as
கண் இமை நொடி என அவ்வே மாத்திரை (The duration of a mora equals the blink of an eye)
See also 
|Look up mora in Wiktionary, the free dictionary.|
- Crystal, David (2008). A Dictionary of Linguistics and Phonetics 6th ed.. Blackwell Publishing. ISBN 978-1-4051-5296-9.
- The Inflectional Accent in Indo-European. Paul Kiparsky. Language. Vol. 49, No. 4 (Dec., 1973), pp. 794–849. Linguistic Society of America.
- Juliette Blevins and Sheldon P. Harrison. "Trimoraic Feet in Gilbertese". Oceanic Linguistics, vol. 38, No. 2, December 1999.