A mora (plural morae or moras; often symbolized μ) is a basic timing unit in the phonology of some spoken languages, equal to or shorter than a syllable. For example, a short syllable such as ba consists of one mora (monomoraic), while a long syllable such as baa consists of two (bimoraic); extra-long syllables with three moras (trimoraic) are relatively rare. Such metrics are also referred to as syllable weight.
The term comes from the Latin word for "linger, delay", which was also used to translate the Greek word χρόνος : chrónos (time) in its metrical sense.
The general principles for assigning moras to segments are as follows (see Hayes 1989 and Hyman 1985 for detailed discussion):
In general, monomoraic syllables are called "light syllables", bimoraic syllables are called "heavy syllables", and trimoraic syllables (in languages that have them) are called "superheavy syllables". Some languages, such as Old English and present-day English, can have syllables with up to four morae.
A prosodic stress system in which moraically heavy syllables are assigned stress is said to have the property of quantity sensitivity.
Further information: Ancient Greek accent
For the purpose of determining accent in Ancient Greek, short vowels have one mora, and long vowels and diphthongs have two morae. Thus long ē (eta: η) can be understood as a sequence of two short vowels: ee.
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 Old English, short diphthongs and monophthongs were monomoraic, long diphthongs and monophthongs were bimoraic, consonants ending a syllable were each one mora, and geminate consonants added a mora to the preceding syllable. In Modern English, the rules are similar, except that all diphthongs are bimoraic. In English, and probably also in Old English, syllables cannot have more than four morae, with loss of sounds occurring if a syllable would have more than 4 otherwise. From the Old English period through to today, all content words must be at least two morae long.
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.
See also: On (Japanese prosody)
Most dialects of Japanese, including the standard, use morae, known in Japanese as haku (拍) or mōra (モーラ), rather than syllables, as the basis of the sound system. Writing Japanese in kana (hiragana and katakana) is said by those scholars who use the term mora to demonstrate a 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. Therefore, scholars argue that the 5/7/5 pattern of the haiku in modern Japanese is of morae rather than syllables.
The Japanese syllable-final n is also said to be 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 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, on this analysis, they can be said to have two, three and four syllables, respectively. The number of morae in a word is not always equal to the number of graphemes when written in kana; for example, even though it has four morae, the Japanese name for Tōkyō (とうきょう) is written with five graphemes, because one of these graphemes (ょ) represents a yōon, a feature of the Japanese writing system that indicates that the preceding consonant is palatalized.
The "Contracted sound" (拗音) is represented by the three small kana for "ya" (ゃ), "yu" (ゅ), "yo" (ょ), these do not represent a mora by themselves and attach to other kana, all the rest of the graphemes represent a mōra in their own.
There is a unique set of mōra known as "special mora" (特殊拍) which can't be pronounced by itself but still count as one mora whenever present, these consist of "nasal sound" (撥音) represented by the kana for "n" (ん), the "geminate consonant" (促音) represented by the small tsu (っ), the "long sound" (長音) represented by the long vowel symbol (ー) or a single vowel which extends the sound of the previous mōra (びょ「う」いん) and the "diphthong" (二重母音) represented by the second vowel of two consecutive vowels (ばあ「い」).
This set also has the peculiarity that the drop in pitch of a word (so called "downstep") can not fall on any of these "special mora" under any conditions, which is especially useful for learners of the language trying to learn the accent of words. The above rule doesn't apply to ん (the nasal N) which for the Japanese doesn't qualify as special and the drop in pitch can fall on ん, for example in the word 日本 (にほん/nihon), where に starts low, the pitch raises and peaks at ほ, then drops at ん and continues low through the following particle if it is present.
In Luganda, 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 Luganda is based on morae. See Luganda tones and Luganda grammar.
In Sanskrit, the mora is expressed 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 (diphthong) ai (which has either two simple short vowels, a+i, or one long and one short vowel, ā+i) is assigned a value of two mātrās. In addition, there is plutham (trimoraic) and dīrgha plutham ("long plutham" = quadrimoraic).
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 dīrgha/guru (दीर्घ/गुरु, "heavy") feet based on how many morae can be isolated in each word. Thus, for example, the word kartṛ (कर्तृ), meaning "agent" or "doer", does not contain simply two syllabic units, but contains rather, in order, a dīrgha/guru foot and a laghu foot. The reason is that the conjoined consonants rt render the normally light ka syllable heavy.